Command & Conquer: Red Alert takes place in a parallel universe. At the Trinity Site in New Mexico in 1946, Albert Einstein prepares to travel back in time. He activates his experimental time machine to find himself in Landsberg, Germany, on December 20, 1924, where he meets a young Adolf Hitler just after the latter's release from Landsberg Prison. Following a brief conversation between the two, Einstein shakes Hitler's hand, which erases him from the timeline.

Hitler's death prevents him from rising to power as leader of Nazi Germany, effectively creating a new timeline. Without Nazi Germany, the Soviet Union (USSR) grows powerful under the rule of Joseph Stalin. The USSR seizes land from China and then invades Eastern Europe, to achieve Joseph Stalin's vision of a Soviet Union stretching across the entire Eurasian landmass. In response, the countries of Western Europe (including an already-rearmed Germany) form the Allied Nations and start a guerrilla war against the invading Soviet Army. Over the course of the game's story, the Allies and Soviets fight for control over the European mainland in an alternate World War II
Extract what factions fight in the video game Command & Conquer: Red Alert
The Allies and the Soviets fight for control over Europe in the game Command & Conquer: Red Alert