If a war broke out and another country (pick whatever one you want) was being attacked, would you volunteer to fight in defense of the country? For the sake of this discussion, we'll assume that your government will not bar you from volunteering and that your country's government refuses to get involved in the war.
Notable examples of people volunteering in foreign militaries include:
-American volunteers helping Canadian republicans in the Canada Rebellions of 1838.
-Foreign volunteers helping the Boers in the Second Boer War.
-American volunteers in both World Wars.
-Foreign volunteers helping Finland in the Winter War.
-Foreign volunteers helping both sides in the Spanish Civil War.
-Foreigners volunteers helping Israel in the Arabi-Israeli War.
-Foreign volunteers helping Rhodesia in the Rhodesian Bush War.