CNN
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Security guarantees. It’s the phrase Ukrainian President Volodymyr Zelensky used countless times during an explosive clash with US President Donald Trump and Vice-President JD Vance at the White House last week – and since.

How can Ukraine be assured that Russian President Vladimir Putin would abide by any ceasefire deal – and not resume fighting in a year or two, Zelensky asks. And how can Ukraine be protected from the unyielding ambitions of its more powerful neighbor?

Trump has been openly dismissive of Zelensky’s preoccupation with such guarantees. “Security is so easy, that’s about 2% of the problem,” he said during Friday’s Oval Office showdown.

Trump’s answers to the wider issue of Ukrainian security have been vague, beyond claiming the Europeans will handle it and that there’ll be no need for a US backstop.

“It should not be that hard a deal to make,” Trump said Monday, hours before he announced a pause in shipments of US military aid to Ukraine.

He also suggested that the presence of American companies exploiting Ukraine’s rare-earths and other minerals would be enough to keep Russia at bay. “I don’t think anybody’s going to play around if we’re there with a lot of workers,” he said.

There were plenty of US companies operating in Ukraine on the day before Russia’s full-scale invasion of 2022.

US Secretary of State Marco Rubio might have a more realistic take. In a Fox News interview last week, he said that “what Ukraine really needs is a deterrent … to make it costly for anyone to come after them again in the future.” He added that this “doesn’t have to just be America. I mean, the Europeans can be involved in that.”

Other US officials have said the US won’t be part of that deterrence. Defense Secretary Pete Hegseth said European troops in Ukraine would not enjoy protection under NATO’s principle of collective security. National Security Adviser Mike Waltz has said the matter of security guarantees is “squarely going to be with the Europeans.”

European leaders met in London on Sunday to begin to search for some answers for Ukraine as well as longer-term solutions to the unravelling of transatlantic relations.

“This is a once-in-a-generation moment for the security of Europe,” British Prime Minister Keir Starmer said, calling for a “coalition of the willing.”

“Europe knows one thing: the deal, if it happens, is not simply about carving up Ukraine or securing a quick ceasefire … it is about a lasting and secure peace agreement, about existential security issues for all of Europe,” Wolfgang Ischinger, a former German ambassador to Washington, wrote in Foreign Affairs.

But Claudia Major and Aldo Kleemann at the German Institute for International and Security Affairs said in a recently published paper that the Europeans “lack both the necessary military capabilities and the political will and unity” to shoulder the burden.

French President Emmanuel Macron optimistically suggested negotiations would take “several weeks and then, once peace is signed, a (troop) deployment.” The deployment would have to be agreed with Russia.

Macron acknowledged, however, that a truce along the 1,000-kilometer (6,200 mile) front line would be “very difficult” to enforce. Peacekeepers would have to operate in a landscape of forests, fields and the wreckage of industrial towns, surrounded on three sides in some areas, with poor or non-existent roads.

The UK and France have expressed a willingness to be part of a post-conflict force to maintain the peace. Australia has also said that it is open to discussing a role.

But the response of other Europeans has been underwhelming. Outgoing German Chancellor Olaf Scholz said it “will require an effort that many are not yet really sufficiently prepared for.” Other allies evaded questions about their readiness to join a peacekeeping mandate. Italian Prime Minister Giorgia Meloni said that deploying Italian troops “has never been on the table.”

Starmer says other countries are ready to contribute but has not identified them. But the UK prime minister also said that the “effort must have strong US backing,” which is far from assured.

Estimates for the size of this proposed force vary as wildly as ideas for its mission and powers. Is it a small tripwire force that deters because it is backed by a more robust response to any violation? Is it a fully equipped mission able to defend itself?

A UN force is unlikely because “Russia, as a concerned state and permanent member of the UN Security Council, would have to agree to a deployment,” Major and Kleemann wrote in a paper, yet to be published and shared with CNN.

But they warn that “a ‘bluff and pray’ approach that deploys too few troops and relies essentially on the hope that Russia will not test it would be irresponsible.”

Zelensky pressed the same point in London over the weekend, insisting on the need for “very specific security guarantees and with very specific providers of these (guarantees)” that would make “100% impossible any kind of opportunity for Russia to come with another aggression.”

A full-fledged peacekeeping force would need to be at least 100,000-strong, an overwhelming commitment for European armies alone, especially when necessary rotations are included. By comparison, the peacekeeping mission that began in Kosovo in 1999 had 48,000 soldiers. Ukraine is more than 50 times the size of Kosovo.

Analysts also say that such a force would require a substantial demilitarized zone (DMZ) separating the combatants and a constant liaison with both sides to handle violations.

There would need to be a Line of Control and the withdrawal of heavy weapons to a minimum distance of 40 kilometers (around 25 miles), they say. And neither side could fly drones in the DMZ.

There is also the issue of drones. Intervening in a conflict where drones and missiles have changed the nature of warfare, the peacekeeping force would require “electronic warfare, counter-drone and counter-intelligence capabilities,” Mick Ryan, the author of the blog Futura Doctrina, said.

There are countless escalation risks, too. If Russian forces were to drop long-range shells on an outpost of French or British soldiers, would that put NATO states at war with Russia? That might even be tempting for the Kremlin. If it targeted European troops near Ukraine’s front lines, NATO’s European members might find themselves at war with Russia without US support.

Meanwhile, a lightly policed ceasefire is unlikely to cut it.

“At best, a highly unstable situation would obtain, where a renewal of hostilities would be easily possible or even likely,” Marc Weller, a professor of international law at the Cambridge Initiative for Peace Settlements, said.

Matthew Schmidt, professor of national security and political science at the University of New Haven, told CNN that realistic security guarantees involve three components: a substantial international presence on the ground, US backing, and a modernized and enlarged Ukrainian army.

Up to 100,000 peacekeepers, alongside a Ukrainian land force of some 200,000 soldiers, might suffice as a deterrent, he said. That would amount to roughly one-third of the Russian force deployed in or around Ukraine.

Starmer insisted that the “effort must have strong US backing.”

Even a well-equipped peacekeeping force would require US airlift capabilities, satellite coverage and missile defenses to deter fresh Russian offensives, all assets the Europeans lack.

Zelensky asserts that the “best security guarantee is a strong Ukrainian army and a strong Ukrainian army that has enough numbers.”

Ukrainian capabilities would also have to include longer-range Western missiles that would allow Ukraine to go after Russian supply lines and logistics hubs, as well as far more potent air power, in the event hostilities resume.

But unless the Kremlin is forced to negotiate, this is all a pipedream. The Russian Foreign Ministry has already said that the presence of NATO troops in Ukraine, whether under the Alliance banner or not, would be “categorically unacceptable.”

Official Russian news agency RIA Novosti quoted the Foreign Intelligence Service on Tuesday as saying that a force of 100,000 peacekeepers “would amount to the de facto occupation of Ukraine.”

Russia “has made maximalist demands and will prove very difficult to budge,” Ischinger wrote. “It is an illusion to believe that a durable peace with Russia will break out simply by enshrining the line of contact in eastern Ukraine.”

Russia’s upper hand?

By Putin’s theory of war, Russia is winning. It’s making incremental gains that may be accelerated if Ukraine loses critical US military hardware. The Institute for the Study of War said Putin’s priorities are to “prevent Ukraine from acquiring and sustaining the manpower and materiel needed to stop gradual but continued Russian advances.”

With Trump in the White House, the Kremlin sees Zelensky as isolated, and the Europeans as left to fend for themselves. Russia has no incentive to compromise on its demands, which include possession of all four eastern regions of Ukraine that have been illegally annexed (even if Russia doesn’t occupy all of them); strict limits on the size and capabilities of the Ukrainian military; and Ukrainian neutrality.

“Ukraine should adopt a neutral, nonaligned status, be nuclear-free, and undergo demilitarization and denazification,” Putin told an audience last summer. By Moscow’s logic, Zelensky must go, because Russia cannot sign a deal with an “illegitimate” leader.

When push comes to shove, would the Trump administration turn the screws on the Kremlin? Trump and other senior US officials have said that Russia will be expected to make concessions, without providing details.

Under international law, recognition of Moscow’s rule over the four eastern regions of Ukraine would break every precedent, so the territorial question would have to be deferred, as has been the case in the Korean Peninsula for more than 60 years.

As the Trump administration indicates that a defense review will lead to an eventual reduction of the American military footprint in Europe, and in the wake of its decision to pause military aid to Ukraine, all eyes are on European leaders’ next moves.

Europe can only contribute to the security guarantees that Ukraine needs through developing its own defense identity, combining joint research, production and training. This will not happen overnight, but steps to resuscitate Europe’s defense industries are already underway. Now they need to be turbo-charged.

The European Commission has been bullish about developing a fund for the defense industries, and on Tuesday, European Commission President Ursula von der Leyen proposed allowing EU countries to draw up to €150 billion in loans and unlocking up to €800 billion of additional defense spending over the coming years.

She said EU members could “pool demand and to buy together and, of course, with this equipment, member states can massively step up their support to Ukraine.” The goal: to turn Ukraine into a “steel porcupine” that proves “indigestible for future invaders.”

Looming over any prospect of a ceasefire deal, however, is the question of Russia’s long-term intentions – whatever the Kremlin might publicly agree to.

The 1994 Budapest Memorandum included an assurance that Russia, along with the UK and the US, would “respect the independence and sovereignty and the existing borders of Ukraine” in return for Ukraine giving up Soviet-era nuclear weapons it had inherited.

Moscow repeatedly undermined the Minsk process designed to resolve the status of territories in eastern Ukraine’s Donbas region that were seized in 2014-15 by pro-Russian militia. Putin persistently insisted Russia had no intention of invading Ukraine, until it did.

Ukrainian Economy Minister Yulia Svyrydenko recalled Monday how the ceasefire deal for Donbas signed by Russia in Paris in 2019 was violated by Moscow within weeks.

“Then, on February 18, 2020, Russians launched one of the largest assaults of the war. This is the Kremlin’s pattern: deception, false promises, and escalation,” Svyrydenko said.

More than Zelensky’s demands for security guarantees, that pattern is the greatest obstacle to peace.