Regarding the ruling of Islam on joining the non-Muslim army of a particular country a Muslim belong to, it is permissible provided certain conditions and regulations are observed.
Sheikh Yusuf al-Qaradawi, a prominent Muslim scholar, said:
“It is allowed for a Muslim to join the non-Muslim army of the country where he was
born or raised. However, there are certain regulations and conditions that have to be taken into one’s consideration:
1. One should not take part in a war that goes against his belief and religion.
2. One should adopt Islamic morals of justice and equity and not to transgress against others.
3. One should practice Da`wah (preaching Islam) whether among his colleagues or among the soldiers of his enemy.”