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.”