WW2 was started by the invasion of Poland. As partners in a pact, both Nazi Germany and Stalin's USSR invaded Poland in 1939 and took control of it for themselves. The USSR and the Nazis remained in alliance until Hitler reneged on the pact and stunned Stalin by invading the USSR in 1941.
Eastern Europe was never "liberated", it ended up under the control of 1 rather than 2 totalitarian powers. At Yalta, an ailing Roosevelt, significantly influenced by pro Soviet advisers like Hopkins and Hiss (a Soviet agent), gave Stalin all that he wanted for the USSR after the German defeat - Churchill had no means to put brakes on Stalin's territorial ambitions without the US, though he wanted to.
Eastern Europe was never "liberated", it ended up under the control of 1 rather than 2 totalitarian powers. At Yalta, an ailing Roosevelt, significantly influenced by pro Soviet advisers like Hopkins and Hiss (a Soviet agent), gave Stalin all that he wanted for the USSR after the German defeat - Churchill had no means to put brakes on Stalin's territorial ambitions without the US, though he wanted to.