President Trump has vowed to end the fighting in Ukraine. Just how he could do that remains unclear, given that President Vladimir V. Putin of Russia believes he is winning. But in his own blunt way, Mr. Trump has opened up the possibility of some kind of negotiations for a cease-fire.
If a deal was to be reached, analysts say, Mr. Trump would be likely to ask Europe to put it in place and to take responsibility for Ukraine, wanting to reduce the American commitment.
But a key question remains: How to secure what is left of Ukraine and prevent Mr. Putin from restarting the war, even several years from now?
The prospect of a deal has accelerated debate over so-called European boots on the ground to keep the peace, monitor a cease-fire and help deter Russia from future aggression. The question is whose boots, and how many, and whether Mr. Putin would ever agree.
It is a topic sure to be a central focus for discussion this week at the annual Munich Security Conference, which Vice President JD Vance and Secretary of State Marco Rubio are scheduled to attend.
Some European countries, among them the nations of the Baltics, as well as France and Britain, have raised the possibility of including some of their own troops in a force in Ukraine. Senior German officials have called the idea premature.
Short of NATO membership for Ukraine, which seems unlikely for many years, the idea of having large numbers of European troops from NATO nations seems reckless to many officials and analysts.
Without clear American involvement in such an operation — with American air cover, air defenses and intelligence, both human and technical — European troops would be at serious risk from Russian probing and even attacks.
President Volodymyr Zelensky of Ukraine has indicated that he is ready for serious talks about a deal to end the war, so long as his allies provide security guarantees, not just assurances.
In the absence of NATO membership, which he prefers, Mr. Zelensky has spoken of as many as 200,000 foreign troops on the ground in Ukraine. But that is nearly three times the size of the entire British army and is regarded by analysts as impossible.
A senior European official said that the continent doesn’t even have 200,000 troops to offer, and that any boots on the ground must have American support, especially faced with the world’s second-largest nuclear power, Russia. If not, they would be permanently vulnerable to Russian efforts to undermine the alliance’s political and military credibility.
Even a more modest number of European soldiers like 40,000 would be a difficult goal for a continent with slow economic growth, troop shortages and the need to increase military spending for its own protection. And it would likely not be enough to provide realistic deterrence against Russia.
A real deterrent force would typically require “well over 100,00 troops assigned to the mission” for regular rotations and emergencies, said Lawrence Freedman, emeritus professor of war studies at King’s College London.
The danger would be a policy of what Claudia Major, a defense expert with the German Institute for International and Security Affairs, called “bluff and pray.”
“Providing too few troops, or tripwire forces without reinforcements, would amount to a bluff that could invite Russia to test the waters, and the NATO states would hardly be able to counter this,” she wrote in a recent paper with Aldo Kleemann, a German lieutenant colonel, about how to secure a Ukrainian cease-fire.
That is why Poland, which neighbors Ukraine and is deeply involved in its security, has so far dismissed taking part in such a force.
“Poland understands it needs the United States to be involved in any such proposal, so wants to see what Trump wants to do,” said Alexandra de Hoop Scheffer, acting director of the German Marshall Fund. “It wants guarantees from Trump that there will be U.S. security help to support Europeans in the front line.”
But that is not at all clear, she said. “Trump will do the deal and look for a Nobel Prize and then expect the Europeans to pay for it and implement it,” she said.
Still, European “willingness to be ready to do something useful” for Ukraine without the Americans will be important to ensure that Europe has a seat at the table when negotiations finally happen, said Anthony Brenton, a former British ambassador to Russia.
Mr. Putin’s stated aims have not changed: the subordination of Ukraine into Russia, a halt to NATO enlargement and a reduction in its forces, to force the creation of a new buffer zone between the Western alliance and the supposed Russian zone of influence.
Nor is it likely that Russia would agree in any deal to the deployment of NATO or NATO- country forces in Ukraine in any case, even if they were ostensibly there to train Ukrainian soldiers. The Russian Foreign Ministry has already stated that NATO troops in Ukraine would be “categorically unacceptable” and escalatory.
Mr. Freedman described three possible models — peacekeeping, tripwire and deterrence — all of which have significant flaws.
Peacekeepers, intended to reinforce agreed-upon cease-fires and keep belligerents apart, are lightly armed for self-defense and often contain troops from many countries, usually under the United Nations. But given that the line of contact in Ukraine is some 1,300 kilometers, or more than 800 miles, he said, “a huge number of troops” would be required.
Before the 2022 invasion, there was an international monitoring mission of the Organization for Security and Cooperation in Europe, with Russian agreement, to supervise observance of a much shorter cease-fire line in eastern Ukraine. It was a failure, said Michael Bociurkiw, who was its spokesman from 2014 to 16.
“The Russians did everything to block the mission,” he said. “They pretended to cooperate, limited access and hid various nefarious activities. When things don’t work the way they want, they shut it down.”
A tripwire force is essentially what NATO has deployed in eight member countries closest to Russia. There are not enough troops to stop an invasion or to be seen by Moscow as provocative, but the concept only works if there is a clear, unbreakable link between the troops on the ground and larger reinforcements committed to fight once the wire is tripped.
But there are always doubts about the absolute nature of that guarantee. And an attacking force would gain significant territory before any reinforcements arrive, which is why NATO itself is increasing the size of its tripwire forces from battalion to brigade level, to enhance deterrence against a newly aggressive Russia.
The third type, a deterrent force, is by far the most credible, but needs to be very large and well-equipped, and would require up to 150,000 well-equipped troops, plus significant commitments of air defense, intelligence and weaponry — and American help with the strategic enablers Europe continues to lack, from air transport to satellites to missile defense.
But it would be hard to imagine that Russia would agree to any such force for precisely the same reasons that Mr. Zelensky wants one, Mr. Freedman said.
So the best answer for the near future after a potential cease-fire may be some version of the “porcupine” model: giving the Ukrainian military enough weaponry, resources and training — including by Western forces — to convince Russia not to try again. Such a commitment, however, would have to be for the long term.
But first Ukraine must stop Russia’s slow advance in the east and Mr. Putin must be convinced to end the war, with further battlefield losses and economic pressure. How to do that will be one of the main tests for Mr. Trump if he is to have success in ending the killing, as he promises to do.
