Americans have volunteered to fight for foreign armies for hundreds of years without repercussion. Most notably, the Spanish Civil War (Lincoln Battalion), WW1 (France, Britain), WW2 (Canada, France, Britain, China) and the various Israeli wars.
Lincoln Battalion in Spain:
If shit ever hits the fan here in Europe (Russia, ISIS, etc.), bet your ass I'd be helping out on behalf of US interests. Military experience goes a long way in volunteer units.