Ukraine can defeat the separatists but it can’t defeat Russia. For Ukrainian President Petro Poroshenko to build a strong Ukrainian state, independent of Moscow, he first must secure a cease fire.

All paths to victory for Kiev require Russia exit the battlefield. The problem is there’s no way for Ukraine to force this militarily.

That’s why Poroshenko should call on NATO leaders this week to commit to a peacekeeping force in eastern Ukraine. That guilt-ridden alliance should jump at the chance to draw a line against Russia’s Putin in Ukraine, where it will have a real deterrent effect. By sending NATO troops into Ukraine as peacekeepers, the West will signal its resolve against Moscow’s imperial intentions, while better securing Kiev’s political and economic future.

Congress should support Ukraine by passing a resolution supporting the deployment of U.S. troops as peacekeepers. President Obama has already taken a half-step, sending 200 soldiers on a peacekeeping exercise. Senators Carl Levin, D-Mich., and John McCain, R-Ariz., and Rep. Adam Smith, D-Wash., have already called for sending weapons to Ukraine. The more effective move would be to send U.S. and NATO peacekeepers.

Funneling weapons to Ukraine also won’t make a difference. Ukrainian troops don’t have time to train with and master the kinds of arms they need to counter Russia. Moreover, Russia ended Ukrainian air superiority by providing the separatists with surface-to-air missiles. Ukrainian troops now operate on battlefields where enemy artillery can move and fire with little risk of being attacked from the air.

The number of separatists is now greater than the number of troops Ukraine can field. And Ukraine says there are 15,000 Russian troops fighting alongside the separatists. At best, Kiev has been able to put 12,000 soldiers into the fight on any given day. To put this into context, when the U.S. Army fights in urban environments, military doctrine calls for a three-to-one advantage at minimum; and that’s with tactically superior troops against second-tier defenders.

Moreover, the mix of “holidayers” from Russian special forces, and separatists (many of whom are Soviet veterans) means the rebels are the more experienced and better trained fighting force. Add in the Russian conscripts being sent into the fight, and the reality of the situation is clear: Moscow is fighting a war of attrition against Kiev.

It’s a war Kiev can’t win. Every time Ukraine puts the rebels on the ropes, Putin simply adds more and better troops to bolster his side  . . .  500 today, 1,000 tomorrow. Even if Poroshenko’s government could match the numbers, Putin will beat him with superior troops and weapons.

This is why NATO needs to put peacekeepers in Ukraine. They’d be a tangible sign of the West’s resolve to halt Putin’s expansionism.

Of course, Putin would almost certainly respond to NATO by putting Russian “peacekeepers” on his side of the line of demarcation. There is risk in this for the West. There will be great unease at having Russian and U.S. forces so close. NATO and Soviet troops spent decades in similar proximity during the Cold War. We can manage the stand-off safely.

Ukraine has become something bigger than itself. It’s a symbol for the West’s core moral interest in democracy and self-determination. If those words mean anything, they’re worth at least as much risk in NATO’s European home, as the alliance has accepted in Afghanistan for far lesser goals.

It won’t be easy for Poroshenko to accept the temporary loss of key land, but a sober assessment of the military balance shows that there’s no military solution for him. He must push for peacekeepers now before the battlefield situation further deteriorates. Inserting western peacekeepers is the best remaining move for Ukraine. It would trigger a freezing of the conflict, but with NATO peacekeepers it wouldn’t give Putin the leverage over Ukraine’s future that he wants. With a strong U.S. presence the hot-war could be frozen be on terms that would safeguard Ukraine’s long-term future. This is why Congress should act now. Both Presidents, Poroshenko and Obama, need the push.

Such a frozen conflict would give Kiev the moral high-ground, and is a step above the faux federalism Russia wants. Ukrainian nationalism would strengthen in the Russian-occupied east as the differences between the Russian and Europeanized zones grew.

The West knows how to win back occupied territories. It’s the model that broke the Soviet Union’s power over a divided Germany. The process may take years, but it stands a better chance of success than pressing the fight against a much stronger enemy.

With NATO and EU help, Ukraine could build a strong economy and the capable military it needs to stand as a bulwark against Russia and ensure its long-term independence.

Dr. Matthew Schmidt is an assistant professor of National Security & Political Science at the University of New Haven. Christopher Shays is a distinguished fellow in public service at the University of New Haven and a former member of Congress, where he was a senior member of the House Homeland Security Commttee and chairman of the National Security Subcommittee of the House Government Reform and Oversight Committee.