The date is already forgotten, but its significance may return to haunt us. On March 29, 2004, Britain promised to fight for Estonia just as surely as we would defend the White Cliffs of Dover. That was the day when the Baltic country joined NATO and fell under the protective umbrella of Article V of the North Atlantic Treaty, obliging Britain and 26 other allies to defend Estonia with “armed force”

Did our leaders grasp the possible consequences of guaranteeing the security of a former Soviet republic with an eastern border only 100 miles from St. Petersburg? At the time, Western governments probably assumed that signing up to fight for Estonia and its Baltic neighbours, Latvia and Lithuania, did not mean very much. After the momentous events of 2014, no one can entertain that comforting illusion any longer. In a Europe where Vladimir Putin has dismembered Ukraine, brazenly annexing 10,000 square miles of his neighbour’s territory, it should be abundantly clear that our pledge to safeguard the Baltic states is a deadly serious undertaking.

Even the briefest visit to Tallinn, the Estonian capital, where I recently was, serves as an antidote to complacency. President Toomas Hendrik Ilves told me of the “snap exercises” staged by the Kremlin near Estonia’s border, involving 40,000 to 80,000 troops. He described how Russian warplanes routinely probe his country’s airspace, adding that Putin’s bombers had practised how to obliterate Sweden and Denmark with nuclear strikes.

As Russia torments Ukraine and the Baltic skies fill with military aircraft, Estonians believe they have every right to be alarmed. The country has conscription and meets NATO’s target of spending 2 per cent of national income on defence. But, in a nation of 1.3 million people, the army consists of 5,300 soldiers. Through no fault of its own, Estonia is defenceless without outside help.

And that is where the facts become profoundly worrying. You might have assumed that NATO forces were already protecting its most vulnerable members. In fact, not a single American or NATO combat soldier is based anywhere east of Germany. Back in 1997, NATO signed a “Founding Act” with Russia promising that no combat units would be permanently deployed on the territory of any new member of the alliance.

Throughout Ukraine’s agony, NATO has kept its word. Today, one company of U.S. infantry with 150 men represents the only foreign contingent in Estonia — and they are only present temporarily. American and British soldiers visit for exercises, but they always go home afterwards. President Ilves believes that NATO’s insistence on binding its own hands is mistaken. He wants the alliance to deter Russia by permanently stationing at least a brigade in the Baltic states.

Yet NATO has turned him down, choosing instead to assemble a “very high readiness task force” of 5,000 troops, which could deploy in a matter of days. Ilves is not reassured by this.

During the Cold War, NATO planners assumed that any Soviet invasion of the West would advance across the Fulda Gap, a flat and tank-friendly expanse of Germany. Today, Ilves thinks the “new Fulda Gap” lies between Russia and its exclave of Kaliningrad. In a war, Russia would grab a corridor of territory east of Kaliningrad, thus blocking every land route to Estonia and its Baltic neighbours. NATO’s “very high readiness” troops would never be able to arrive. “Great idea,” is the president’s verdict on this force. “But it probably is, in terms of the realities, just too late.”

So here are the sombre facts: no American or NATO soldiers are permanently defending the Baltics; these countries could not protect themselves; if the worst happens, NATO would not be able to reinforce them. Does this mean that Ilves is right and NATO should permanently station a brigade while it has the chance?

The problem is that Russia would regard this as a grave escalation. Before you start climbing the escalatory ladder, you must be sure that your adversary will not go three or four rungs higher. It’s safe to assume that Putin would always be willing to climb further than the West, so Russia would inevitably win a game of escalation.

Where does this leave us? During the Cold War, NATO assumed that a Soviet offensive through the Fulda Gap could only be defeated by nuclear weapons. But the permanent presence of 200,000 U.S. troops in Germany — along with 55,000 soldiers from the British Army of the Rhine — would slow down the onslaught and buy a few days, or perhaps weeks, for cooler heads to prevail before the terrible moment of decision arrived

Today, no such safety margin exists. If Russia were to invade the Baltic states, NATO would probably have one option — and one alone — to defend its members. America, Britain and France would need to decide almost immediately whether to use nuclear weapons. If they opted to abandon the Baltics, then NATO would be finished.

Once a collective defence pact throws one member to the wolves, the game is up. At that moment, NATO would effectively be dissolved, leaving every European country with no choice but to ask Russia for gentle treatment. By moving against the Baltic states, Putin could force us to choose between scrapping NATO or going nuclear.

Does anyone believe the thought has never crossed his mind?