Reason is slipping from the left. Not on economics; it is perfectly sensible for Jeremy Corbyn’s People’s Army to campaign over re-nationalising the railways or taxing the billions of pounds hidden offshore. But on foreign policy the left is giving into a Ukipish, quack Little England agenda of pulling the country out of the Nato alliance.

This is a dangerous idea. Nato is Europe’s peace framework: the military and diplomatic hardwiring that stops old bloodlands in Central and Eastern Europe from fearing new wars. It is not a capitalist vampire, but the architecture that underwrites the stability of the EU.

This is not a view from Washington; it is the view from Prague, Warsaw and Tallinn, from Paris, Berlin and Stockholm. It is the position of all EU parties of government.

Nato works because people believe in it as a guarantee from the strong militaries – above all the US, UK and France – to keep the region stable. Central Europeans passionately believe Nato is responsible for maintaining peace in the Balkans, that it keeps Greece and Turkey from each other’s throats, that it stops old rivals such Hungary, Romania and Slovakia from vendettas over one another’s territory. Above all, Nato keeps the Baltic states safe from Vladimir Putin’s “interventions”.

Wanting out of Nato is not the same from opposing Nato expansion, or opposing the historic fact of its creation. But that is very different from Corbyn’s 2020 exit plan.

Taking the UK out of Nato would be disastrous. Not only is Britain’s army so materially super integrated it would soon struggle to function, our exit would pointlessly destabilise Europe – sending a very clear message to Putin and Russia.

British exit would derail German-led peace talks over Ukraine; Putin would certainly see the West as weak and push deeper in. Where would he stop? Many in the Kremlin argue a divided, weak, declining Europe will not stand up to Russia if it conquered Ukraine’s sprawling Black Sea coast. Britain out of Nato strengthens the position of these militarists.

The bottom line in handling the Kremlin is this. Even if you want a neutral Ukraine, remember Putin only respects hard power. Labour’s apparent incoming leader shows no signs of listening to EU democracies. Corbyn’s thinking on Nato sounds like the views of a man who has spent too much time watching and appearing on Russia Today.

Across Central Europe, national leaders have shared their fears with me: that Putin’s wars of aggression will continue to the backdrop of a fraying Nato. The number one worry in Warsaw and Tallinn is that Vladimir Putin will decide that Nato is a paper tiger and repeat his “hybrid war” on Ukraine – staged uprisings and cloaked special forces whipping up civil war – in Russian-speaking towns in the Baltic. Late at night they even talk of Poland fighting a Baltic invasion alone one day, if Nato deterrence fails. When this is what allies fear, for Labour to advocate pulling Britain out of Nato in the name of peace is letting the fantasy politics of a North Islington dinner table override reality.

Let’s imagine Corbyn does become Prime Minister, and a horrified German Chancellor and American President double down on Eastern Europe as a result. This manoeuvre will only have achieved one thing – the final and full conversion of a rich and powerful state into an isolated eccentricity.

Corbyn’s grand gesture will see Britain treated with nothing but contempt – by the martial states we wish influence, from Israel to Iran, and by the French, Germans and Americans, who will see our foreign policy agenda as a cretin’s act of disloyal sabotage.

There is nothing admirable about a Labour bid to build this Red Little England here, listened to by none on any matters of war or peace. Who will this help other than Putin? The Palestinians? The Ukrainians? Voiceless, weak states help nobody.