Command & Conquer: Red Alert 2

From the introduction:

When Albert Einstein traveled back in time to eliminate Hitler before the Nazis rose to power, the professor’s hope was that this change would prevent the horrors of the Nazi regime. He succeeded, but in its place came a different, equally devastating conflict. Joseph Stalin, without Hitler’s power to stand in the way, sought to lay claim to all of Europe in a rather different Second World War. He lost, but the course of history was changed forever.