Labour is collectivist by instinct and culture. Founded by the trade unions, it could hardly be otherwise. The welfare state the Attlee Government did so much to build was founded on collective insurance. Labour has stood in solidarity with oppressed groups, minority communities, countries under attack and many others in the past.

Collective security is solidarity by another name. It’s wholly fitting, therefore, that the Attlee Government took the United Kingdom into NATO as a founding member. As the Cold War deepened, western Europe needed the United States to guarantee its security. Not all NATO’s members were always democratic, but it nonetheless bound free Europe to the US. Since then, it has formed the bedrock of British defence policy. Article 5 of the North Atlantic Treaty provides almost our ultimate insurance – our fallback in an existential crisis.

When the Iron Curtain lifted and the Soviet Union fell, new eastern European democracies wanted to join NATO. Given their history, it’s hardly surprising. On a continent where collective security lost its meaning, Chamberlain and Daladier browbeat Edvard Beneš into signing away the Sudetenland in September 1938. Czechoslovakia was dismembered by Hitler the following March. A Franco-British guarantee meant nothing. In September 1939, Britain and France did honour their guarantee to Poland. Poland still suffered years of unspeakable horror. In 1945, eastern Europe came under Soviet control: the ‘people’s democracies’ only fell in 1989. Again, the West stood aside. You might say the West had little alternative in 1945-8: it’s a sobering and unedifying story nonetheless.

That so many of those countries have now joined the community of Western democracies is a cause for celebration. Churchill’s ‘capitals of the ancient states of Central and Eastern Europe’ no longer languish behind an Iron Curtain, but have joined with their sisters to their west. NATO has provided the anchoring for a community of states linked by law to grow together.

The last Labour Government was a good friend to eastern Europe – championing its right to a place in the Western world. And until now, no Labour leader has ever wavered in their support for the Atlantic Alliance. Michael Foot opposed nuclear weapons, but he kept Labour pledged to NATO. Jeremy Corbyn’s ambivalence at present and in public, and his hostility in the past and probably in private, is unique. His opposition to NATO deployment in eastern Europe as a deterrent to a revanchist Russia is deeply misguided. His refusal to say he would defend NATO allies under attack is profoundly dangerous.

In 2017, eastern Europeans have good cause to value their NATO membership. In February 2014, Russia and Estonia signed an (admittedly unratified) agreement finalising their border. In March, Russia invaded and annexed Crimea because Ukraine wanted to sign a deal with the European Union. Both countries have large Russian minorities. Both once formed part of the USSR. Granted, Ukraine is central to Russia’s sense of self in a way which doesn’t hold true for the Baltics (of course, this doesn’t make its aggression any better). But another crucial difference is the guarantee under Article 5.

It would be unwise enough in normal circumstances to disdain our current insurance policy with no alternative in mind. In these abnormal ones, it is almost farcical in its foolishness. In 2014, a great power annexed a neighbour’s territory on the continent of Europe for the first time since 1945. Its leader is hostile to the liberal international order on which the UK relies, and nothing in his history suggests ‘relationship resets’ or a pacifying stance will appease him. It has links to far right (and radical left) parties in democratic Europe. It props up a toxic but anti-Western regime in Syria. It interfered in the US presidential election and may have even affected the outcome.

Thanks in no small part to the US election, NATO has rarely faced greater threats from within. In eight days’ time, a man whose commitment to European security is questionable will become President of the United States. That Donald Trump and Theresa May agreed on the importance of NATO in a telephone call is of limited comfort, even if he meant what he said. Credibility means everything in deterrence: it matters that Trump once said he wouldn’t mind too much if NATO dissolved, not just whether he continues saying such things as President. This is exactly the moment at which European governments must try to ensure NATO does not wither on the vine. It’s also a moment when NATO’s word must be seen to be its bond. I struggle to imagine a worse time to argue for undermining its shared deterrence strategy in Europe.

History makes clear that Britain cannot ignore the rest of Europe, that its security is bound up with its continent. Brexit does not change the essential fact, however much Nigel Farage might like it to. If the US disengages, Europeans will need to look to our own security: the UK has no opt-out. But in any event, it would be profoundly wrong to let Putin dictate policy in eastern Europe – and even more so to regard any NATO ally as somehow dispensable. I see nothing left-wing about old-style spheres of influence. I see nothing progressive in ignoring eastern Europeans’ right to choose their own destinies. Estonia, Latvia and Lithuania became liberal democracies by their own free choice. They are our allies. They deserve better than to be treated as our buffer states.

Collective security is vital for democratic Europe. It has not faced such severe threats from without for many years. It has rarely, if ever, seemed more besieged from within. Britain could never separate itself from the fallout if it broke down. And even if it could, it would be an appalling act to abandon our friends and allies to Vladimir Putin. No true progressive should countenance it.