France, Poland, and the British entered a defensive pact before the war began. For everyone else, to enter into an alliance with the Allies meant--for the most part--that they were entering into the war until hostilities against the Allies ended.
There was also a matter of differing ideologies. The fascists involved with the Axis powers were viewed as making war against capitalism (UK, US, France) and communism (USSR, China). There was a shared understanding that if the fascists were to win, free societies and communist societies alike would be at risk of extinction.
