A hundred years ago, a thin strip of land that constituted the western Polish region of Pomerania separated two non-contingent areas of Germany - the mainland of the Weimar Republic in the west and eastern Prussia with its capital Königsberg. Perched in between was the free city of Danzig.

After the second world war, Königsberg was annexed by the Soviet Union and renamed Kaliningrad after a Bolshevik revolutionary. And once again, this city is separated from its mothership by a corridor, known as the Suwalki Gap - a 65km long strip of flat land along the Polish-Lithuanian border, named after a Polish town south of the border. The gap mattered little during the Soviet times, when Lithuania and Belarus were Soviet republics and when Poland was part of the Warsaw Pact. It mattered little in the immediate aftermath of the break-up of the Soviet Union in 1990. But as Vladimir Putin and Alexander Lukashenko took steps last week towards a political union, this gap could become another source of geopolitical tension. In the north-south direction it separates Kaliningrad from Belarus. In the east-west direction it constitutes the only land connection between the Baltic Republics and the rest of the EU.

One of the lessons of European history is that corridors are bloody dangerous. When we heard Vladimir Putin accusing the west of staging a military coup in Belarus to topple  Lukashenko, we listened up because European land grabs are usually preceded by fake allegations and staged incidents. Hitler’s invasion of Poland was preceded by the so-called Gleiwitz incident, an attack on a German radio transmitter - allegedly perpetrated by Polish soldiers, but actually carried out by the Nazis themselves.

Tensions between Belarus and Poland have been rising. Belarusian security forces recently arrested members of the Polish community in the south-western Belarusian city of Brest, which triggered Polish accusations that Belarus was persecuting the Polish minority. Lukashenko accused the Polish government of sheltering run-away traitors, as he called them. Earlier this month, Poland deployed troops from the 18th division of the First Warsaw Armoured Brigade to the border in direct response to the crackdown on the Polish minority in Belarus.

Last week, Putin and Lukashenko agreed a step-by-step process towards eventual political union. Many western commentators seemed relieved that Putin did not drop a diplomatic bombshell during his state of the union address last week. But Putin is actively cultivating stories of a western plot to assassinate Lukashenko. “The practice of organizing coups and planning political assassinations of top officials goes over the top and crosses all boundaries,” he said. I am wondering whether we are heading towards a modern-day Gleiwitz incident.

When Hitler invaded Poland, one of his goals was to reclaim territory lost after the first world war and to reconnect with the cut-off provinces. His invasion was supported by the German populations of Danzig and Königsberg.

Seven years ago, before Russia annexed Crimea, the Sevastopol city council, controlled by pro-Russian parties, accused Ukraine of a terrorist plot. The population of Crimea voted overwhelmingly in favour of secession from Ukraine, and annexation to Russia.

It is not hard to construct a scenario that could eventually lead to military confrontation in the wider Polish-Lithuanian-Belarus border area.

Ben Hodges, the former commander of the US army in Europe, coauthored a detailed study in 2018 of what it would take to defend the Suwalki gap. The study noted that this is a hard-to-defend area. It has only two narrow highways and one railway line. It would not take much for the Russians to cut off the Baltic Republics.

It would be naive to think that Nato would immediately trigger the Art. 5 solidarity clause. I see no chance whatsoever that Germany would ever participate in military action against Russia - even if Russia invaded the Baltic Republics. Just consider the words of Frank-Walter Steinmeier, the German president, who said Germany owed the Nord Steam 2 pipeline to Russia to atone for its sins during the second world war. He once described Nato exercises in the Suwalki gap as sabre-rattling. I see no majorities in German politics for any form of military action.

Hodges recommends a strategy of what he called preclusive defence - not to wait until Russia acts, but stop Russia from acting in the first place. That strategy requires an upgrade of military hardware and a stronger Nato presence in the region. But it is still reliant on an Art. 5 trigger. I think this is the weakest link in the western strategy. The more dependent Germany and other EU countries become on Russian gas, and Russian vaccines, the less likely they will be willing to confront Russia. I think we have long passed the point where this is politically possible.

Putin knows this too. He is playing a long game. The political union with Belarus is part of a long-term strategy. It won’t happen overnight, but phased in successively over a period of a few years.

Remember that he once described the collapse of the Soviet Union as a genuine tragedy. Putin is no Hitler. He won’t reclaim the entire geographic area of the Soviet Union. But he claims security interests that extend beyond Russia’s immediate borders. A weak and divided EU and Nato would further those interests.

There is a logic to what he is doing right now. That logic leads me to conclude that the Suwalki gap is potentially a very dangerous place.