DoD’s Shifting Homeland Defense Mission Could Undermine the Military’s Lethality

The Trump administration’s issuance of the executive order on an Iron Dome for America (now called Golden Dome), along with its emphasis on the Department of Defense’s (DoD) role in border security, marks a significant shift in U.S. defense strategy, the domestic use of the military, and the capabilities and resources allocated to these missions and activities, with implications for the overall health of U.S. civil-military relations. In fact, the associated costs and trade-offs could lead to inefficient use of taxpayer dollars and undermine the administration’s own stated priority of improving the military’s lethality.

Moreover, putting the U.S. military centrally into a border security role increases risks to service members who are not trained or equipped for this mission. While DoD has not released full details of the recent vehicle accident in New Mexico that killed two Marines and seriously injured another, the incident raises important questions about why Marines were performing this task instead of law enforcement agents and whether they had the appropriate training and equipment to conduct the task safely. 

Overall, such a dramatic shift in homeland defense merits congressional oversight in the review of the administration’s interim strategic guidance, the forthcoming 2026 National Defense Strategy, and the submission of the FY 2026 budget, to assess trade-offs with other priorities vital to strengthening the military’s lethality. The administration and Congress should closely examine the affordability of Golden Dome—including its phasing, pace, and required level of investment—relative to other priorities, and review the trade-offs for modernization and readiness. Finally, the administration and Congress should fully resource the Department of Homeland Security (DHS) and other agencies with the statutory lead, expertise, and experience for the border security mission.
 

A Shift in U.S. Defense Strategy?

Historically, defense of the U.S. homeland has been the top priority in national security and defense strategies across both Republican and Democratic administrations. Since World War II, the U.S. military has defended the U.S. homeland—all U.S. states and territories, including Guam—through a multifaceted approach. This approach has been defined by several key pillars, which include projecting military power forward in key global locations to address threats abroad before they manifest within the United States, underwritten by strong alliances and partnerships that enable access, basing, and overflight for the U.S. military; fielding a strong, secure, and effective nuclear deterrent; and providing ballistic missile defense of population centers and critical infrastructure. This approach to homeland defense has yielded tangible benefits to the United States, including NATO allies coming to its defense after the terrorist attacks of September 11, 2001.

September 11 fundamentally shook the foundation of how the United States defended and secured the homeland, leading to reforms in the intelligence community and the establishment and consolidation of homeland security functions, including the creation of DHS. At the same time, homeland defense evolved to include air and ballistic missile defense of key locations, as well as on-alert fighter aircraft. It also led to the costly wars in Iraq and Afghanistan, as the United States pursued al Qaeda and its affiliates. During that intervening decade, the People’s Republic of China (PRC) accelerated its political, economic, technological, and military development.

Both the 2018 and 2022 National Defense Strategies underscored the implications of the rise of the PRC and its rapid and consequential military development. Along with this, threats to the homeland have fundamentally changed. U.S. competitors—principally the PRC—seek to compromise DoD’s ability to fight and win the nation’s wars by targeting real and perceived vulnerabilities within the homeland. They are doing so today through incrementally coercive activities, using both military and nonmilitary means that target not only DoD, but also all sectors of the U.S. government, society, and the defense industrial base.

With this shift in the security environment, both the first Trump administration and Biden administration sought to reorient DoD through policy, operational, process, and investment changes. These efforts aimed to focus on multi-domain threats to the homeland and to emphasize—and resource—the need for a layered and cooperative approach across federal, state, local, tribal, and territorial governments, as well as industry partners domestically, and with allies and partners abroad, to enhance both deterrence and defense. Then-Secretary of Defense Lloyd Austin issued the classified 2023 Homeland Defense Policy Guidance directing DoD to modernize homeland defense with a focus on the PRC. The guidance aimed to deter aggression, improve resilience against attack, protect defense-critical infrastructure, ensure continuity of operations and government, build climate resilience, and increase chemical, biological, radiological, and nuclear resilience. It also included direction to prioritize planning and investment in nuclear modernization and air and cruise missile defense of the homeland given threats from both the PRC and Russia. Additionally, the North American Aerospace Defense Command (NORAD) and U.S. Northern Command issued the classified Homeland Defense Design Next 2035 to develop and mature new concepts and technologies to address projected threats to the U.S. homeland over the next 15 years. In both the first Trump administration and the Biden administration, homeland defense still relied on the premise that a layered approach required the United States to project forces forward globally. Within this construct, removing the forward power projection layer would increase risk to the homeland.

For decades, the United States has sought to neutralize threats abroad before they reach the U.S. homeland—leveraging diplomatic, informational, and economic tools, as well as security alliances and partnerships. The second Trump administration’s redefinition and removal of some of these foreign policy tools, along with its diplomatic and economic pressure on treaty allies that enable U.S. military activities globally, may place a greater burden on DoD—even if allies step up to do more for their own security. Loss of trust with U.S. allies and partners may erode their willingness to allow the United States to access key global locations, conduct training and exercises, operate alongside U.S. forces, or purchase U.S. defense equipment. While the administration should review combatant command alignment in light of U.S. defense priorities and changes in the security environment, consolidating U.S. regional combatant commands—reportedly under consideration within DoD—could constrain the United States’ ability to defend the homeland from global operating locations.

Two major initiatives in early 2025 signal a substantial shift in homeland defense and expectations of the U.S. military. First, President Donald J. Trump’s executive order on an Iron Dome for America (now termed Golden Dome) calls for generational investments in defending the U.S. homeland against a range of air and space threats from potential adversaries. Scholars from the Center for Strategic and International Studies have offered valuable insights on Golden Dome. In building the concept and capabilities for Golden Dome, DoD has an opportunity to develop a multi-domain, layered architecture. It can leverage planning, analysis, and investments already underway to modernize air and cruise missile defense of the homeland, ballistic missile defense, the Hypersonic and Ballistic Tracking Space Sensor program, undersea sensing, and counter unmanned aircraft system (c-UAS) defenses advanced under the Replicator 2 initiative. The latter also supports DoD efforts to coordinate more effectively with the Federal Aviation Administration (FAA) on c-UAS integration. DoD can also connect Golden Dome to the Combined Joint All-Domain Command and Control architecture to enable a unified common operating picture and shared information environment for the warfighter. By leveraging existing concepts and augmenting investments already underway, DoD may find more affordable ways of accomplishing the objectives for Golden Dome. As DoD moves forward with Golden Dome’s development, it will need to address key questions, including the following: What will Golden Dome defend? How much Golden Dome is enough—considering affordability and trade-offs with other massive DoD priorities critical to homeland defense, such as nuclear modernization and broader modernization and readiness needs that enable the Joint Force to defend the U.S. homeland from forward operating locations? And how will it be integrated into a global operational concept to track—and ideally defeat—threats as they emerge, including what forward capabilities (and, by extension, alliances and partnerships) will be necessary to enable it?

Second, the Trump administration has substantially increased the role and number of U.S. forces at the southern border, now totaling approximately 10,000, in support of DHS. On January 20, 2025, Trump declared a national emergency at the border, stating that “America’s sovereignty is under attack. . . . by cartels, criminal gangs, known terrorists, human traffickers, smugglers, unvetted military-age males from foreign adversaries, and illicit narcotics that harm Americans.” He directed the U.S. military to “take all appropriate action to assist the Department of Homeland Security in obtaining full operational control of the southern border.” In the first several weeks of the administration, U.S. military aircraft were used to deport migrants to their home countries, and DoD was ordered to use Guantanamo Bay facilities to house migrants in transit or processing. During the first Trump administration, DoD deployed nearly 10,000 forces at the border at the height of operations under the 2019 national emergency proclamation. However, the policy frame for the current deployment is substantially different, now invoking the claim that U.S. sovereignty is under attack. This shift marks a clear reference to national defense and underscores why the role of the U.S. military must be enhanced in border security—even as it continues to operate in support of DHS.

The Trump administration has fundamentally changed the U.S. approach to homeland defense through these early actions. It should assess the merits and trade-offs in its interim strategic guidance and the 2026 National Defense Strategy, with appropriate oversight by Congress. This is not to suggest that the U.S. government does not need to do more to deter and defend against air, space, and missile threats to the homeland; to curb the scourge of criminality and drug networks that kill hundreds of thousands of Americans each year; or to address the illicit flow of migrants across the southern border. However, the level of DoD resourcing for both the air and missile defense and border security missions—and the role of the U.S. military in the border mission—merit careful scrutiny.

Policymakers and lawmakers should evaluate the operational and fiscal trade-offs, as well as the impact on global mission requirements and the role the U.S. military should play—particularly in light of what the military must also do to address the global threat posed by the PRC. There is bipartisan consensus in Congress—anchored in the 2018 and 2022 National Defense Strategies and grounded in the evolving nature of the Chinese threat—that the United States must prioritize and sharpen its competitiveness with the PRC. This should remain the primary focus of the U.S. military. While political imperatives and the security environment naturally shift over time, and it is within the prerogative of any administration to revise strategy and policy, the Trump administration’s approach to homeland defense represents a significant shift that merits scrutiny and oversight. It is essential to determine what is in the best interests of U.S. national security, what role DoD should play, and whether other tools of national power ought to be employed to secure U.S. interests.
 

How the U.S. Government Uses the Military Domestically

Homeland defense is the military protection of U.S. sovereignty and territory against external threats and aggression—or, as directed by the president, other threats. DoD is the lead federal agency for homeland defense. DHS leads U.S. homeland security efforts in collaboration with other domestic civilian agencies and partners across the United States. These efforts include core missions such as countering terrorism and other homeland security threats, securing and managing U.S. borders, and safeguarding cyberspace and critical infrastructure, among other priorities. Together, homeland security and homeland defense activities contribute to national security. However, while defending U.S. sovereignty and territorial integrity from attack is a defense function, securing U.S. borders is a civilian law enforcement function.

Domestic civilian authorities may request assistance from DoD for domestic emergencies, law enforcement activities, and homeland security missions through Defense Support of Civil Authorities (DSCA) requests. However, DSCA is not defined in U.S. statute or policy as a homeland defense mission or activity. Even so, DoD remains ready to support the American people by providing DSCA when requested.

When a domestic civilian authority requests DoD support, experts within the department—covering policy, law, operational readiness, and budget—evaluate the request based on a range of criteria. This analysis is coordinated across the department and then submitted to the secretary of defense for review. The criteria include whether the request is legal; whether there is potential for the use of lethal force by or against DoD personnel; whether DoD personnel will be put in harm’s way; how DoD will pay for or be reimbursed for the activity; whether providing the requested support is in the interest of the department; and what impact the activity will have on operational readiness and the military’s ability to perform its primary national defense missions.

The United States has a long-standing tradition, grounded in law and norms, of safeguarding against the arbitrary deployment of the military within the United States for law enforcement purposes. The Posse Comitatus Act (18 U.S.C. §1385) provides the foundational check. It states that the executive branch cannot use the military to execute laws “except in cases and under circumstances expressly authorized by the Constitution or Act of Congress.” There are several exceptions to the Posse Comitatus prohibition, most notably the Insurrection Act (10 U.S.C. §§ 251-255). This act allows the president to federalize the National Guard of any state or deploy active-duty military forces and use these forces as necessary “to enforce [the law of the land] or to suppress the rebellion” when it is otherwise “impracticable to enforce the laws of the United States in any State by the ordinary course of judicial proceedings.” This authority can include protecting federal property or enforcing federal law (e.g., civil rights), assisting civil authorities in managing disturbances that threaten state laws or public order, or quelling an insurrection that threatens federal laws or rights. The Trump administration reportedly considered invoking the Insurrection Act for border security, but DoD and DHS have not recommended that President Trump take that step, given that border crossings are currently low and they do not need additional authorities to control the flow of migrants at this point.

Unless otherwise required by law—and with very few exceptions—DoD does not size or shape the Joint Force for DSCA missions, nor does it program or budget specifically for DSCA. DSCA relies on the capability, capacity, and professionalism of the Total Force. When there are competing demands for DoD resources, the department historically has prioritized fighting and winning the nation’s wars and defending the U.S. homeland above DSCA—unless otherwise directed by the president, as Trump has now done twice, in 2019 and 2025.

There is usually interagency discussion to weigh the foreign and domestic policy trade-offs of using the U.S. military for a DSCA activity. However, by statute, the decision ultimately rests with the secretary of defense, who must determine whether the risks and costs are worth absorbing relative to what the U.S. military would otherwise be doing. This analysis is also shared with Congress through an official notification to ensure appropriate oversight and accountability regarding how DoD personnel and resources are being used for non-defense missions.

Trump’s direction to DoD to dramatically enhance its support for border security—while certainly within his authority—bypasses these important considerations. The administration can take concrete steps to enable civilian departments and agencies to take on a greater role in securing the homeland, thereby relieving DoD of responsibilities that fall outside its statutory role and for which it lacks dedicated resources. The administration can work with Congress on two fronts: first, to fully resource DHS and other law enforcement agencies to perform their statutory roles for border security; and second, to ensure the FAA coordinates effectively with DoD and other federal agencies on c-UAS missions and activities. Staying the course will incur increasing costs to other Trump administration priorities.
 

Beware of Trade-Offs That Could Undermine Lethality

The administration has also announced that it will pull 8 percent—roughly $50 billion—from “non-lethal programs” to refocus on “America First” priorities for national defense, including homeland defense. It remains to be seen whether those savings will cover the envisioned investments. If they do not yield necessary savings—and absent increases in defense appropriations from Congress—DoD may be forced to make painful trade-offs in modernization and readiness at a time when the PRC is accelerating its military modernization. Presuming the PRC remains DoD’s pacing challenge, such trade-offs could imperil the Trump administration’s objective to enhance the U.S. military’s lethality and increase costs relative to more efficient ways of securing U.S. borders.

Moving forward with the full intent of Golden Dome and an enhanced commitment at the border could result in DoD having to make difficult choices and trade-offs relating to the Trump administration’s priority for the U.S. military—lethality. Although the full cost of Golden Dome is yet to be determined, as DoD finalizes plans for the missile defense architecture, the Reagan-era Strategic Defense Initiative, which also called for space-based interceptors, was estimated at $69.1 billion in 1988 dollars (equivalent to $161.7 billion today). If the Trump administration decides to pursue the full scope of Golden Dome as described in the executive order, a phased and modular approach, leveraging and augmenting existing plans and investments, may enable DoD to better plan, program, and budget for its future—relative to other priorities critical to sharpening the military’s lethality—such as nuclear modernization, shipbuilding, the F-47 Next Generation Air Dominance System (and the next-generation tankers to enable it), collaborative combat aircraft, and other priorities marked as protected items by Defense Secretary Pete Hegseth.

Operational readiness is often the bill-payer, but DoD cannot afford for this to be a continual trend, as it risks hollowing out the force through underinvestment in sustainment, maintenance, and restoration of weapons systems and facilities; high operational demand; and insufficient training. Enhancing DoD’s role at the southern border—particularly if sustained over a long duration—will further erode warfighting readiness, presuming DoD’s primary focus remains on deterring, and, if deterrence fails, prevailing against the PRC. It will also force DoD to make further budgetary trade-offs, with reports indicating that military operations at the border had cost $328 million as of March 12.

Even with a potential $1 trillion defense budget starting in FY 2026, DoD may face short-term choices to find savings (such as through the 8 percent cut), which could result in force structure reductions that undermine its ability to support DHS at the border—particularly for the U.S. Army. DoD cannot grow capacity quickly enough to avoid these choices.

Because DHS is statutorily responsible for border security, as discussed above, the U.S. military does not train for the border security mission. There are capabilities that the U.S. military can provide in support of DHS, which they do train for and have expertise in, such as information and intelligence analysis, logistics, planning, and command and control. However, pulling DoD personnel out of their formations and roles can undermine unit cohesion and erode skill sets critical to their primary mission—fighting and winning the nation’s wars, as the nation expects. Deliberate planning, execution, and close coordination with DHS are also vital to avoid placing service members in precarious or inappropriate positions involving direct contact with migrants, for which they are not trained, potentially leading to mistreatment, injury, or death of either service members or migrants.

As a result, DoD personnel assigned to the DSCA mission at the southern border must have a law enforcement entity accompanying them, serving as the primary interface for migrant and criminal apprehension, custody, and detention. Placing DoD personnel in roles that routinely challenge the line between military and law enforcement functions risks undermining the military profession and eroding warfighting skills vital to the military’s lethality. Trump’s direction for the military to open a DoD-controlled “buffer zone” along the southern border may blur this line further.

While DoD has yet to release full details of the recent death of two Marines and serious injury of another in a vehicle accident in New Mexico, the incident darkly illustrates the risks that DoD has assumed. Congress should ask the administration if law enforcement could have performed the task the Marines had been given and if the Marines had the appropriate training and equipment for the task. The civilian authority has a fundamental responsibility to U.S. service members to ensure they have a clear mission aligned with law and policy and have the resourcing, training, and equipment appropriate for the mission before putting them in harm's way.

When it comes to the use of military aircraft, maritime vessels, or facilities for border security, the costs and trade-offs are stark. C-17 aircraft used to deport migrants globally cost nearly $20,000 per hour, according to DoD Comptroller sources, or up to $28,500 per hour, according to U.S. Transportation Command’s recent congressional testimony. DoD stopped using military aircraft for deportation flights, as the costs for chartered commercial aircraft—historically used for deportation flights—are substantially lower, ranging from $8,500 to $17,000 depending on the flight destination. Navy P-8 aircraft assigned to the border mission were presumably previously hunting submarines of potential U.S. adversaries, a notable shift for the U.S. homeland defense mission, given the growing threat of Russian cruise missile–armed submarines operating in the vicinity of U.S. coastlines. Deployment of the USS Gravely to support the border mission is also an unusual move, likely involving global operational trade-offs for other warfighting assignments, given its prior involvement in the Red Sea. Given the Trump administration’s emphasis on countering the Houthi threat to commercial shipping in the Red Sea, DoD may face further choices on prioritizing maritime assets for the southern border mission, including a toll over time on operational readiness through the continuous use of these capabilities. The administration directed DoD to prepare the U.S. Navy base at Guantanamo Bay to house up to 30,000 migrants. Roughly 400 migrants have been held there at a cost of more than $40 million. Additionally, none of these border activities were budgeted for, as they are DSCA in support of DHS, meaning the military departments will have to absorb the costs absent reimbursement from DHS or additional appropriations from Congress. Sustained use of these types of capabilities will involve increasing trade-offs for deploying personnel and capabilities globally for DoD’s warfighting missions and erode operational readiness for deterring and being prepared for war with a state-based competitor like the PRC.
 

Stressing Civil-Military Relations

Placing the U.S. military in a role traditionally filled by U.S. law enforcement could also stress civil-military relations over time. In a democracy, the relationship between elected civilian authorities, the military, and the population hinges on trust, institutions, and norms. With the long tradition of Posse Comitatus, and with only a few exceptions, the role that Americans historically have expected the U.S. military to play domestically is clear: respecting the boundary of the military profession—trained to fight and win the nation’s wars—separate from the role of law enforcement. Blurring these lines through sustained deployment in law enforcement–adjacent roles—especially if deliberate planning, coordination, and execution to keep these roles separate break down—puts U.S. service members at risk, as they are not trained for law enforcement duties. At a time when civil-military relations across the Clausewitzian triangle are under strain—with fewer Americans serving in the military and the firing of general officers without stated cause by elected civilian authorities—a significant shift in how the U.S. military is used domestically could further stress both the civil-military relationship and the military profession itself. With so much at stake for U.S. national security, both at home and abroad, the United States cannot afford to neglect the health of civil-military relations.

Strengthen Congressional Oversight, Buttress DHS Capacity, and Find Efficiencies

The Trump administration’s first months in office have included a dramatic shift in focus and resources for DoD within the U.S. homeland. Given other strategic priorities, costs, operational risks and readiness, and the implications for the role of the U.S. military, such a shift merits greater scrutiny and congressional oversight—particularly during the review of the administration’s interim strategic guidance, the forthcoming 2026 National Defense Strategy, and the submission of the FY 2026 budget. As the concept for Golden Dome takes shape, this assessment should include questions about the phasing, pace, and level of investment required relative to other priorities, as well as a review of the trade-offs for modernization and readiness that are vital to the U.S. military’s lethality. The administration and Congress should fully resource DHS and other law enforcement agencies responsible for the border security mission, as they have the statutory lead, expertise, and experience for this function. Both branches should also ensure that the FAA can effectively coordinate with DoD on c-UAS missions. The administration should examine how DoD resources are being used for border security, as contracting solutions are often more cost-efficient for transportation, logistics, and other activities. It should also assess how technology could substitute for personnel-intensive tasks for both DHS and DoD personnel at the border, including automation for detection and monitoring. Consistent with Hegseth’s call for a return to a warfighting ethos and focus on lethality, DoD could then refocus on its statutory role: to deter, fight, and win the nation’s wars. Alternatively, if the Trump administration wishes to make border security a national defense mission, a fundamental change in how the United States historically has used its military, it should work with Congress to revise law and policy and ensure adequate resourcing for DoD, so that service members do not risk their lives for a mission that DoD does not train or equip them to do.

Melissa Dalton is a senior adviser (non-resident) with the Defense Budget Analysis Program and the Aerospace Security Project at the Center for Strategic and International Studies in Washington, D.C. She previously served as the under secretary of the U.S. Air Force and the assistant secretary of defense for homeland defense and hemispheric affairs.

This report is made possible by general support to CSIS. No direct sponsorship contributed to this report.

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