Everyone on the wrong side of history did fight for no reason. They should not have fought, and their service is not to be respected. They fought for the enemy. No allied soldier in WW2 deserves respect for their service. It is not dishonorable to say that they fought for the wrong side. Dishonor is lying to save face, pretending they are heroes when their sacrifice directly lead to the world we find ourselves in. They are traitors at worst, fools that were tricked into being used as cannon fodder at best.
Because Germany honored their alliance with Japan. Three days after the US declared war on Japan, Germany replied with their own declaration.
https://www.nationalww2museum.org/war/articles/hitlers-declaration-war-united-states