Before dawn, 1 September 1939. A German battleship opens fire on the Polish depot at Westerplatte, and armies cross the border along its whole length. On 17 September the Red Army comes in from the east. A secret protocol signed in Moscow on 23 August has already divided the country.
Why then
Hitler wants Danzig and the Polish corridor that cuts East Prussia off from Germany, and a war while the window looks open. Stalin, kept out of Munich, takes a deal: half of Poland and a free hand in the Baltic. Britain and France have guaranteed a Poland they cannot reach.
What they wanted
Berlin needs a short war and a quiet east. Moscow is after the eastern half of Poland without fighting Germany yet. Poland fights to exist, and holds out five weeks.
What changed
Britain and France declare war on 3 September and do almost nothing for months. Both occupiers set out to destroy Poland's educated class: the Germans shoot teachers and priests, the Soviets deport hundreds of thousands and in 1940 shoot some 22,000 Polish officers and officials at Katyn and elsewhere.
Still argued
Polish historians say the alliance promised more than it ever meant. Could France have struck the thinly held German west in September 1939? Its generals thought not; many historians think a real offensive would have hurt.