What should the Anglo-Americans have done about Ukraine?

Just to be completely clear my sympathies are entirely with Ukraine and I have no sympathy with the Russian invasion. 

What should the Anglo-Americans have done about Ukraine? 

They should not have provoked the invasion in the first place but having done so weeks before the invasion they should have sent troops to the Ukrainian-Russian border. 

What should they do now? Insist the Ukrainians accept a deal largely on Russian terms but allowing Ukraine to keep her coast. 

Russia needs a guarantee that the Ukrainians will not launch an attack to recapture their lost territory once Donald Trump leaves office. The Ukrainians need a guarantee that the Russians will not come back for more territory in the future, though I don't think this war is about territory. It's about neutralizing what the Russians perceive as a threat from Ukraine. 

It does seem an insoluble problem, like so many problems in history that lead to wars. It will be resolved on the battlefield with Ukrainian defeat, I imagine.