Okay so if I land US trooper on a land zone that has Italian troops on it and one german soldier but I’m not at war with Germany, this is not a valid combat move until I’m at war with Germany.
If US troops attacked an Italian territory that also has German units in it, you would basically be declaring war on Germany at that point. In land battles, all Allied units defend together so there is no way not to have war between US and Germany.
I think that going strictly by the rules you are supposed to declare war on a nation before making your combat move, but I think most players simply take the combat move as the DOW without making a formal declaration.
You have an interesting situation there with the US being at war with Japan and Italy but not Germany. I can’t help but wonder how well that would work if you kept it up, but I don’t think it would work out. Germany would simply put an extra infantry in each Italian territory and you would never be able to fight the Italians without attacking Germany as well. Then again, in most of our games, Germany and Italy are pretty separated. Italy usually goes up against the Brits in the Med and Africa while Germany dives into Russia.