Follow ups:

Why would they send them specifically to help the U.S.? Why not just the entire Allied war effort?

Why would the U.S. need to "accept" these soldiers? Mexico was and is a sovereign nation, able to send its troops where it wants to.

No one needed U.S. approval to fight in the war. Lots of countries fought. Indeed, most were already fighting the war before the U.S. entered it later on.

I'm not trying to break any rules, but this questions drips with the American chauvinism.