If those efforts fail, Washington will have to decide whether to use lethal force, in the event of another Russian invasion.
Diplomatic sources believe the most likely response would involve air power, reviving an idea once proposed by the coalition of the willing as a “backstop” to deter attacks on any European deployment.
Under the plan, American F-35 fighter jets, Tomahawk missiles or similar systems stationed inside Nato territory could be used to launch counter-strikes if Moscow were to breach an armistice.
In theory, US forces could assist Ukraine by carrying out cross-border deep strikes against Russian military equipment used to sustain a renewed invasion.
Pre-emptive strikes, however, are considered unlikely because of the risk of escalation.
American military action would come only after responsibility for any flare-ups had been clearly attributed to Russia.
US heavy weapons would be the icing on the cake of commitments already on the table from European partners.
The coalition of the willing, led by Britain and France, has proposed doing much of the lifting through the deployment of a multinational “reassurance” force.
Its primary role would be to train Ukraine’s post-war military, use warplanes to police the skies, and secure key Black Sea shipping routes.
A statement signed by more than a dozen European leaders this week said the force would “assist in the regeneration of Ukraine’s forces, in securing Ukraine’s skies, and in supporting safer seas, including through operating inside Ukraine”.
European sources acknowledged that ground forces could also serve as a “tripwire” to slow a Russian invasion before reinforcements arrive.
European nations, however, still have not reached a decision over the “rules of engagement” should their forces come into contact with Russia.
Ukraine held out for US guarantees
The coalition’s offer to Ukraine has been on the table for many months as US President Donald Trump’s peace talks rumble on.
But Kyiv has been holding out for a key American role in the security guarantees. In his latest offer, Trump has given President Volodymyr Zelenskyy and his negotiators what they wanted.
The security guarantees also pledge that Ukraine’s armed forces will be restored and developed into what has become known as a “steel porcupine” – something indigestible to future invaders.
Yesterday, Mark Rutte, Nato’s Secretary-General, described Ukraine as the “first line of defence” during a meeting of the Ukraine Defence Contact Group, a coalition of 50 nations supporting Kyiv’s military needs.
“We still need to ensure Ukraine’s armed forces have the strength to resist any Russian aggression now, of course, but also in the future,” he said.
A 20-point peace plan sketched out between Europe and the US is understood to propose limiting Ukraine’s military to 800,000 service personnel, a compromise made to address Russian demands.
However, there would be no restriction on what weapons systems Ukraine could use to defend itself.
Russian President Vladimir Putin has previously insisted that it should not be allowed missiles that are capable of striking Moscow or St Petersburg.
America is unlikely to directly contribute to the donation of such weapons or finance Ukraine’s domestic defence industry.
The security guarantees are likely to see the continuation of Rutte’s Purl scheme, under which European nations buy US-made weapons to be donated to Ukraine.
The scheme has been primarily used to purchase Patriot air defence systems, but could become a platform to finance long-range missiles, such as Tomahawks.
Increasing doubt that Putin will agree
The scale of the security guarantees offered by the US may have been celebrated in Kyiv and other European capitals.
But work remains to be done in the coming days, with further talks in Miami this weekend.
With Article 5-like guarantees on the table, European sources believe there is increasing doubt that Putin will agree to the proposals.
After all, the Russian President has repeatedly claimed his invasion was mounted because of Nato expansionism.
Western governments, using declassified intelligence, have argued that Putin’s only objective was to topple Ukraine’s European-facing Government.
While Ukraine will not be joining the Western military alliance any time soon, the offer of security guarantees will provide a huge strategic roadblock to Putin’s ambitions to bring Kyiv back into Moscow’s sphere of influence.
For Europe, this leaves one question: “How will Trump respond if Putin rejects a peace deal based on these guarantees?”
Zelenskyy argued: “I believe the US will apply sanctions pressure and provide us with more weapons if [Putin] rejects everything. I think that would be a fair request from us to the Americans.
“In my view, the logic is this: if the US is ready to provide security guarantees to Ukraine and to apply strong guarantees if Putin violates the agreement, then tell me honestly – how is that different from a situation in which Putin does not want to end the war?”
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