President Barack Obama's radical revision of Washington's planned missile defence system has left US allies in central and eastern Europe feeling vulnerable.
Washington must now take urgent action to ensure the move does not leave the impression it no longer cares about central and eastern Europe's security - or the region's well-founded concerns about Russia.
Mr Obama's decision was correct. The hugely expensive missile system's backers failed to prove it could work. But they succeeded in irritating Moscow, which, understandably, saw the proposed siting of bases in Poland and the Czech Republic as a provocation.
The way forward is tricky. The new administration's priorities lie in Iran, Afghanistan and the Middle East, and in fighting terrorism and nuclear non-proliferation. Mr Obama is right to court Russia in the hope of winning co-operation on these vital issues. So is Nato.
But the US and Nato must not forget about long-standing fears among Russia's neighbours. The region's largely peaceful transition from communism to democracy was a triumph. If this is compromised, what hope is there of extending the frontiers of freedom elsewhere?
Russia has legitimate interests in neighbouring states, notably in energy. But its authoritarian rulers have often pursued these interests in ways that threaten their neighbours' sovereignty, notably in Georgia and Ukraine. Moscow may now be tempted to exploit what it sees as a tactical victory over the US on missile defence.
The west must minimise the dangers. Having accepted that Nato enlargement is, for now, off the agenda, it must focus on boosting the security of existing alliance members. The US should deploy a planned American-manned Patriot missile unit in Poland. Nato should expand military exercises in eastern Europe, especially in the Baltic states. Meanwhile, Nato's newer members must, despite the economic crisis, implement long-overdue military modernisation.
The European Union, too, has a role to play, notably in energy, where eastern Europe depends on Russian gas. To prevent gas crises of the kind that struck last winter, the union needs more interconnecting pipelines and stores. Energy security is a vital part of overall security.
None of this needs be anti-Russian. It is a matter of maximising the region's security. The safer Poland and other states feel, the more likely they are to support broader co-operation with Russia.