Moe, Larry and Curly never stuck out as particularly competent at anything they did, but in the shorts where they serve in the military they tend to have combat roles. How did they ever get past basic training?
America was headed into The Big One; they didn't have the luxury of being able to refuse anyone.
Its not just WW2, they've served in other military forces, like the one where they're southerners during the Civil War and keep switching sides. The same thing applies though, in all cases they were conflicts where beggars couldnt be choosers.
These were the Stooges. Their commanding officers must have sent them into combat just hoping to be rid of them, permanently. Those three even made their sergeant in basic training cry.