Was entering WWII a mistake?

[Snowstalker Mk. II]

I'm under no illusions that the Western Allies went to war with fascism with noble intentions--an autarkic Germany dominating Europe would be very much contrary to Britain's foreign policy goals for the past 400 or so years, and France was obviously concerned with self-preservation first and foremost--remember that both countries saw Germany and Italy as minor threats compared to the USSR until the occupation of Czechoslovakia in 1939. While Japan did indeed strike the US and its Pacific colonies first, the tensions leading to the Pacific front came from conflict for markets in Asia, with Japanese militarism posing a major threat to both the European colonial powers and American interests in China and its own colonies.

Despite this, the utterly abhorrent nature of the fascist states and the carnage they brought upon the people of Europe and Asia mean that despite my disagreement with the unspoken motives of the western democracies, to go to war against fascism was ultimately for the best.

[Snowstalker Mk. II]

As a revolutionary socialist, surely the obvious answer would be "yes we should have smashed the fascists", no?

That is indeed my final answer (as I stated in the post). Consider my opinion concurrent rather than majority.