IT WAS a single remark in an interview with the New York Times, but it rattled the strongest military alliance the world has ever seen. On July 20th Donald Trump declared that should the Baltic states be attacked by Russia when he is president, he would come to their aid only if he felt they had met their “obligations”. That would contravene Article 5, the bedrock of NATO’s founding treaty, which holds that an attack on one member is an attack on all. NATO’s secretary-general, Jens Stoltenberg, soon pushed back: although he did not wish to interfere in an American election, he said, “solidarity among allies is a key value for NATO.” Russia’s threat to the Baltic states is not notional. Under Vladimir Putin, Russia is fast modernising its armed forces, building a hard-hitting, flexible military that can be deployed at short notice into what it calls its “near abroad”. Russian war games regularly simulate attacks on Estonia, Latvia, Lithuania and Poland. The 2009 Zapad exercises included rehearsing the use of tactical nuclear weapons.

Russia does not want to reconquer the Baltics. But it might want to use the hybrid-warfare tactics it employed in Ukraine (such as disinformation, political subversion, cyber-attacks and the use of special forces without insignia) to demonstrate that the West is reluctant to defend its most vulnerable allies. If, for example, Russia sent forces into a Russian-speaking area of Estonia, NATO would be faced with an existential dilemma: to fight back and risk nuclear war, or to capitulate and destroy its own credibility.

Recognising the danger, NATO has tripled the size of its rapid-response force to 40,000, established a spearhead force that can be deployed within hours and, at its Warsaw summit earlier this month, agreed to put multinational battalions into the four front-line states. The Obama administration is quadrupling spending on America’s military presence in eastern Europe next year. But the deterrent value of all this depends on the world believing that America cares about Europe, and that NATO can make decisions fast. It is faith in these that Mr Trump has undermined.

The three Baltic states are model allies. Latvia and Lithuania are on course to reach the alliance’s target of spending 2% of GDP on defence. Estonia has always hit the 2% mark. Yet Newt Gingrich, tipped as a possible secretary of state in a Trump administration, dismisses it as a “suburb of St Petersburg”. As Toomas Hendrik Ilves, Estonia’s president, pointed out, no Americans have died for Estonia, but Estonians have already died for America: its forces fought enthusiastically in Afghanistan and Iraq—and suffered the highest casualties, per head, of any alliance member.

Mr Trump is right that many European countries spend too little on defence. But to suggest ignoring treaty obligations is reckless. The whole point of a deterrent is that if it is credible, you don’t have to use it. If Mr Trump wins, America’s allies everywhere may feel they are on their own.