Why continue the war against the Soviet Union?

Why would the Western Allies attack the Soviets after defeating Germany?

Opposition to Soviet-imposed one-party states offers a possible motive, but the Japanese war and unwillingness to keep fighting weigh against it.

Opposition to the one-party states developing in the Soviet sphere provides a possible ideological motive for a further war. The scenario depends on the Western Allies placing enough weight on that objection to keep fighting. They also have an unfinished war against Japan and other commitments. War-weariness weighs heavily against prolonging another massive conflict.