WHEN is an invasion not an invasion? When you can do very little about it. Nato figures and politicians were still using the language yesterday of “if” a Russian boot steps across the border into Ukraine… but boots already have.

Sajid Javid was right when he said that “the invasion of Ukraine has begun” and not just in a figurative sense. Sending “peacekeeping” troops into the disputed regions of Luhansk and Donetsk and declaring them “independent republics" is a violation of Ukrainian sovereignty and a flagrant breach of international law.

The fact that there may have been some Russian military presence in the region since 2014 is beside the point. Britain has sent military personnel to Ukraine, but that doesn’t mean we are occupying Kyiv. This is an annexation by Vladimir Putin. The seizure by force of a region of another country just like Georgia and the Crimea.

Putin has torn up the 2015 Minsk Protocol, which he said he wanted implemented. That agreement envisaged devolution to the Russian-speaking territories of the Donbas. They were still to be recognised as part of a sovereign Ukraine.

Bizarrely, the Russian President appeared on the cusp of getting Minsk endorsed. President Macron had acting as honest broker. A summit had been all but agreed between President Biden and President Putin. But instead of home rule Russia has opted for tank rule.

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What madness. It’s like the EU sending peace-keepers to the disputed territory of Scotland because we oppose Brexit. Putin is not acting rationally. By provoking war in Europe he has created what he said he feared most: a massive Nato military build-up on Russia’s frontiers.

Putin is an old man in a hurry. In power for more than 20 years, he wants to make his mark on history by reversing the 30 years of hurt since the end of the Cold War. But the echoes of his annexations go back further than that.

The similarities with Adolf Hitler’s military expansionism in the 1930s are now too obvious to ignore. I loathe the drawing of facile comparisons with the Nazis. But we really have seen nothing quite like this in Europe since the Anschluss and the annexation of the Sudetenland.

Hitler too claimed that ethnic Germans had been denied their nationality, then by the 1918 Versailles Treaty. The German dictator, who was of course initially elected, had strong domestic support for restoring Germany’s “true” borders and uniting the German-speaking peoples.

So Boris Johnson was right to say this is the most dangerous moment since 1945. All the more so because the West is not going to go to war over the dismantling of Ukraine, much as we ignored the dismantling of Czechoslovakia in 1938.

President Zelenskyy may be a former comedian but he made a deadly serious appraisal of the balance of power. He knows the West is not going to fight. So he is wisely urging his citizens not to oppose Russia with force. Keep calm and carry on. Don’t give Putin a pretext for invading Kyiv. With three-quarters of the entire Russian army on his borders he knows that resistance is futile.

But the West will now have to act and not just fulminate. Kicking a few oligarchs out of London is not going to cut it. If Nato means business it will have to broker a sanctions regime that excludes Russia from the global economy in much the same way as pariah states like Iran and North Korea. Deny access to the international banking system and the protocols for international payments. Halt technology transfers and freedom of movement. Cut Russia off from its sources of revenue: gas, oil and other lucrative natural resources.

Sanctions are doable where military force is not. The entire Russian economy is barely the size of California and can be brought to heel in time. But this is going to hurt us all if it is to succeed.

Oil is already over $90 a barrel and will rocket past $100 (though wisely the SNP isn’t cheering). With the tripling of gas prices the global energy crisis is about to get a lot worse. Europe gets 40 per cent of its gas, and Germany 60%, courtesy of Putin. The German Chancellor Scholz has paused the Nord Stream 2 pipeline. But he must go further.

There is a risk that a serious sanctions regime could plunge the post-pandemic world into an economic crisis.

But the danger of doing nothing is that Putin might decide that other Russian-speaking communities, for example in the Baltic, need to be restored to Mother Russia. Latvia, Estonia and Lithuania are Nato countries. If Russia invaded them we would be required by treaty to retaliate. That could lead to nuclear war.

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It is worth taking a moment to reflect on how extraordinary it is to see a country being annexed in the 21st century. We are supposed to be living in an increasingly borderless age; a new paradigm of globalisation presided over by liberal capitalism. War is bad for business, everyone knows that. It disrupts trade, wrecks investment, leads to economic hardship for both sides. No one has taken seriously the prospect of a war in Europe since the collapse of the Soviet Union 30 years ago.

We’re all capitalists now. We’re all caught up in the world wide web and interconnected by inconceivable volumes of international finance. This was supposed to have rendered territorial wars of aggression obsolete. Unfortunately, Putin is an analogue authoritarian in a digital age.

His extraordinary speech on the eve of the annexation of the Donbas was like something from the 19th century. Indeed, his concept of Historic Russia dates not from the Soviet era, but the age of the Tsars when Ukraine was part of the Russian Empire along with the Baltic States. His grievance dates from Brest Litovsk, 1917, when the Bolshevik Foreign Minister, Leon Trotsky, relinquished large parts of the Russian Empire in order to take the newly-communist Russia out of the Great War.

Boris Johnson is right. Putin cannot be allowed to win. And he surely won’t. His macho posturing is anathema. Time to put him back in his box.

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