In 1939, Britain and France went to war to protect Poland from the German and Soviet Union invasion that abrogated the Polish sovereignty the British and French had underwritten.
At that time, the Soviets and Germans were allied with each other against Poland, under the terms of the Molotov-Ribbentrop Pact.
Neither Britain nor France saw Germany or the USSR as existential threats to themselves. They acted in accordance with their duty as underwriters of Polish independence.
By the end of 1941, the situation was radically different, with France defeated and Britain under clear existential threat. Britain needed any help she could get. The Japanese attack on the USA, and then the German betrayal of the USSR, led to both the US and the Soviet Union becoming allies of the British, in a war whose aim had radically shifted from the restoration of Poland, to the destruction of the Third Reich, Imperial Japan, and Fascist Italy, and the liberation of the vast territories they had by then invaded.
Poland and the rest of Eastern Europe was shamefully abandoned to the USSR, from whose yoke she was not freed until the 1990s.
That the US and Britain did not persue Polish independence in 1945 is understandable; War against the USSR in 1945 was undesirable on a huge number of levels, and the war aim of 1939 a distant memory, hidden behind the vast destruction that had been wreaked across the whole of Europe.
But to once again abandon those countries that were abandoned to Russia from 1945 to 1990, including not only Poland and other Warsaw Pact nations, but also the former Soviet Republics of Latvia, Lithuania, Estonia, and Ukraine, would be a further betrayal of a Western European and British historical duty to protect their independence - a protection that is today in the strategic interest of both the EU and the UK, both economically, as free trade is beneficial to all parties; And militarily, as a buffer zone between Putin's armies and German, French (and ultimately even British) territory.
The twin threats to Europe that existed in 1939, the Third Reich and the USSR, have both now been eliminated. But neither Russian expansionism nor Western European fascism are completely vanquished.
Defending Ukraine and the rest of Eastern Europe against Russian irredentism, and the whole of Europe against resurgent ultra-nationalism, are an ongoing (and likely eternal) duty for all who value freedom and prosperity over dictatorship and squalor.
We have both a moral imperative, and a strategic interest, in giving Ukraine all the assistance we can, not only to defeat and repel the current Russian invasion, but also to become a liberal democracy with strong protections against corruption, so she can become a prosperous trading partner for the rest of the free world.
The world needs to see, once again, that invasion and subjugation of neighbouring countries is intolerable, and demands resolute, timely, and unstinting action to ensure that the invader does not profit from his agression.
This is not somebody else's problem. It's our problem. And until we address it, the cost of addressing it will just keep rising.
In 1936, the French and British could have easily occupied the Rhineland to prevent the militarisation of that region. In 1938, they could have made a stand against the occupation of the Sudetenland, instead of signing the craven Munich Agreement. But they didn't act until it was too late, and the cost of defeating the expansionist dictator in Berlin became astronomical.
Let us not allow the cost of defeating the expansionist dictator in Moscow to become similarly inflated.