Approximately what percentage of people supported pacifism during WW2 in the USA? I'm not interested just in the opposition to war, since some people could protest against US involvment from ideological affilation towards Nazism, but how big a role did actual pacifism play?
There were multiple causes for people to oppose US involvement in WWII. They included:
- The political position that the US should stay out of European wars. This was perfectly respectable, and did keep the US out of the war until the Japanese attack on Pearl Harbour, and Germany's declaration of war on the US. The US had had some involvement before then, in selling arms on terms that made it possible for the UK and France to buy them, but much harder for Germany and Italy, and in defending merchant ships in the western Atlantic against submarine attacks, which were all German. This position is linked to, but not the same as, the Munroe Doctrine.
- Actual support of Nazism. This died out once the US was participating in the war.
- Actual pacifism, objection to all wars. There were fake pacifist organisations under communist control as well as real ones.
One useful number in trying to determine how many people were motivated by pacifism is the number of Conscientious objectors who were drafted for non-combatant service, or refused to be drafted. About 72,000 men tried to register as objectors, and 52,000 had this status recognised. Of those, 12,000 went to civilian work camps, over 25,000 served in the military as non-combatants, and nearly 6,000 went to prison, presumably for refusing those options.
Extrapolating from men of draftable age and fitness to the whole population of the USA is going to be tricky, so you aren't going to get exact numbers. About 16.4 million people served in the US armed forces during WWII, so the rate of conscientious objection was quite low.