WW2 was fascist infighting and the moderate fascists sided with the communists to prevent their working population from revolting and creating a reason for the Soviets to continue rolling tanks west to liberate whatever resistance forces appeared.
That and a healthy dose of realpolitik due to Germany attacking France for basically no reason. If they just pushed east, the Western European governments would have probably avoided joining for as long as possible.
WW2 was fascist infighting and the moderate fascists sided with the communists to prevent their working population from revolting and creating a reason for the Soviets to continue rolling tanks west to liberate whatever resistance forces appeared.
That and a healthy dose of realpolitik due to Germany attacking France for basically no reason. If they just pushed east, the Western European governments would have probably avoided joining for as long as possible.