In these dangerous times, Donald Trump has made an elementary error. By announcing that the United States will suspend the Intermediate-Range Nuclear Forces Treaty, he has played into Vladimir Putin’s hands.

Instead of standing firm with America’s allies in Nato against Russian violations of the treaty, President Trump has decided to pull out of the agreement altogether in six months if Russia does not comply. This gave President Putin the excuse to announce today that Russia, too, will abandon the treaty.

Mr Putin can now present Russia and the US as equals, and equally responsible for the collapse of an important deal signed in 1987 that has helped to slow the arms race by banning mid-range nuclear-capable missiles.

This moral equivalence is false. Russia is violating the treaty by developing and deploying missiles specifically banned by it. There has been a debate in Nato about how to respond, with most members seeking intensified economic and diplomatic sanctions against Russia.

Now Mr Trump has swept aside all such talk, and any idea that the US should work with allies to present a united front against Russian provocations. Nato has had to go along with the US president’s unilateral decision, because most of its members understand the value of unity, even if Mr Trump does not. Thus Gavin Williamson, the UK defence secretary, has joined with the rest of Nato in “fully supporting” the decision.

We hope, however, that the British government continues to press behind the scenes for an alternative approach – and at least there are six months in which to do so.

The extent to which Mr Trump’s instincts fly in the face of conventional opinion in America and in Europe was laid bare by James Mattis when he resigned as US defence secretary in December. “One core belief I have always held is that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships,” he wrote. “While the US remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies.”

Friday’s decision by the president confirms that he takes a different view – one which is not just dangerous and destabilising, but also tactically inept. It allows Mr Putin to avoid being held to account for his breach of treaty obligations.

Worse than that, it opens the way for Mr Trump and Mr Putin, by tit-for-tat repudiations that are the very opposite of the kind of patient confidence-building needed for effective diplomacy, to unravel the whole series of nuclear arms control treaties that have made the world a safer place over decades.