Whatever the outcome of the range of bilateral and multilateral talks surrounding Ukraine, one thing is clear: any lasting peace will require a demilitarized zone (DMZ) backed by an international military force. This zone could stretch over thousands of kilometers, traversing complex and hazardous terrain, including bombed-out cities and mine-laden fields. To ensure security, this force must not only act as a deterrent but also have the warfighting capability to counter potential Russian fait accompli attacks along the frontier. And the international force would need to be large enough to support ongoing military training missions integrating Ukraine deeper into the transatlantic security architecture.
Based on analyzing past military missions, the number of foreign troops required to support Ukrainian sovereignty, and security could range from a tripwire force of a battalion to a more doctrinal-sized force of over 100,000 soldiers. And this force will require additional air, naval, and space assets to cover air and maritime corridors Russia could use to launch a future preemptive attack. The security force required to truly safeguard the peace in Ukraine could be as large as the entire military of either Greece or Spain. In other words, peacekeeping in Ukraine has the potential to eclipse previous NATO missions in the Balkans in both its size and complexity.
Troop Density: Why Force to Space Ratios Matter
The ratio of troops to space—or troop density—is an enduring concept used to estimate requirements for military forces. In a 1960 article, B.H. Liddell Hart—the British military theorist—claimed none other than T.E. Lawrence told him to “do a study of the ratio of force to space in war, his own conclusions being that it was of basic importance and contained the clue to many of the puzzles of military history.” In the study, Hart concluded that it would take a minimum of 20 mobile divisions to cover NATO’s frontier with the Warsaw Pact. This topic also drove debates about the conventional balance in Europe and how many armored division equivalents were required to hold back an invasion.
Estimating the Frontier Force
Ukraine’s border with Russia, including the land and the Sea of Azov, is roughly 2,295 kilometers (1,426 miles). That is comparable to the distance from the outskirts of Washington, D.C., to Denver, Colorado. Given Minsk’s subservient role to Russia and use as a staging base, any force would also need to cover the 1,084 kilometer (673 miles) land border between Belarus and Ukraine. There is also over 1,000 kilometers (621 miles) of Black Sea maritime boundary.
In other words, there will likely be over 3,000 kilometers (1,864 miles) of frontage depending on how the territory settles in any ceasefire that requires military observers and peacekeepers. This territory could grow to include additional deployments in Poland and Romania given documented instances of Russian missiles and drones passing through their territory.
Along the land boundary, there would also need to be a degree of depth to observe Russian forces and buy time for a response. The Korean DMZ is only 4 kilometers (2.4 miles) wide, far too narrow given modern weapon advances that see even platoon-sized formations operating small drones up to 32 kilometers (20 miles) from their position. This means that the land portion of any DMZ could range from 12,000 kilometers to 96,000 square kilometers (7,456 to 59,651 square miles) based on an estimated front of 3,000 kilometers.
The Requirements for International Force
Establishing a credible security force in post-conflict Ukraine requires more than symbolic ceasefire monitors. The size and composition of this force will depend on its mission—ranging from military observers to an active conventional deterrent force capable of preventing renewed aggression. Historical peacekeeping missions offer useful benchmarks, from small UN observer teams to large NATO-led stabilization forces. Given Ukraine’s vast borders and the threat of Russian incursions, any security force must balance manning forward observation posts with the mobility and firepower required to prevent spoiling attacks and incursions all too common between 2014 and 2022.
The smallest possible mission size would be internationally recognized ceasefire monitors. For example, the UN Mission for the Referendum of Western Sahara has only 245 observers operating in an area that covers roughly 47,000 square kilometers (29,204 square miles) or 0.005 troops per square kilometer. Applying that to Ukraine produces a requirement ranging from 60 to 480 soldiers. The UN Military Observer Group in India and Pakistan has 109 personnel deployed in ten field stations along the line of control in Jammu and Kashmir, covering roughly 1,600 square kilometers (1,000 miles) of rugged terrain or 0.068 troops per square kilometer. Applying that to Ukraine produces a requirement ranging from 816 to 6,528 soldiers. Since these would be monitors tasked with only reporting ceasefire violations, they would be more of a tripwire than a true deterrent.
More realistic estimates emerge from previous deployments in Lebanon and the Balkans. The UN Interim Force in Lebanon has a total of 14,000 uniformed personnel operating in 50 positions between the Litani River and the Blue Line, which covers 1,060 square kilometers (658 miles). This produces a troop density of 12.18 deployed personnel per square kilometer. Applying that to Ukraine produces a requirement of at least 145,200 soldiers. In December of 1995, NATO deployed the 60,000 strong Implementation Force (IFOR) to secure the peace agreement brokered in the Dayton Accords and safeguard over 50,000 square kilometers in Bosnia-Herzegovina. Unlike most UN forces, IFOR consisted of combined arms formations capable of deterring Serbian aggression while stabilizing the post-conflict space, not just observing and reporting ceasefire violations. The troop density was 1.17, which applied to Ukraine implies a force ranging from 14,040 to 112,320 soldiers depending on the depth of any DMZ.
Peace Through Strength
Ukraine requires more than a token peacekeeping presence—it demands a combat capable international military mission capable of deterring aggression, securing a vast frontier, and integrating Ukraine into the transatlantic security architecture. Historical precedents suggest that anything less than a robust multinational force, backed by air, naval, and space assets, risks failure. The size and structure of this force must balance deterrence with long-term training missions to build Ukraine’s self-sufficiency. As NATO and allied nations debate the future of European security, they must recognize that protecting any peace deal in Ukraine will require sustained commitment, strategic depth, and a force built for endurance.
Benjamin Jensen is the Director of the Futures Lab at the Center for Strategic and International Studies in Washington, D.C. and the Frank E. Petersen Chair at the Marine Corps University School of Advanced Warfighting.
