What is the ruling on a man joining the army of a disbelieving country if he is required, after four months, to swear an oath to defend it in order to continue?

A Muslim is not permitted to enlist in the army of a disbelieving state due to the corrupting influences inherent in such an act. Among these are: exposing oneself to danger, not for the sake of upholding the word of Allah, and aiding disbelievers in fighting one another, which is fundamentally impermissible. This is because fighting must be for the exaltation of the word of Allah and the strengthening of His religion, not for the exaltation of polytheism. Al-Sarakhsi mentioned that Muslim musta'minin (those granted safe conduct) in a land of war should not fight alongside the people of war, unless they fear for their own lives and act in self-defense. Muhammad ibn al-Hasan stated that Muslims should not fight alongside disbelievers against other disbelievers, because both groups are the party of Satan. It is narrated in Al-Mudawwana that Imam Malik disliked Muslim prisoners of war fighting alongside a king of disbelievers against his enemy, and he held that it is impermissible for a Muslim to shed his blood in order to move disbelievers from one form of disbelief to another.