The fifth question of Fatwa no.
Is it permissible for a male or female Muslim to work in the disbelievers' munitions factories,
participate in any activity which strengthens their army in number and equipment, as a computer scientist or the like, or join their armies?
It is not permissible for a Muslim to work in the disbelievers' munitions factories, participate in any activity that increases their power, or join their armies. In fact, this is a sort of Wala' (loyalty) to them. Allah, the Exalted, states,
Let not the believers take the disbelievers as Auliyâ (supporters, helpers) instead of the believers
Moreover, He, the Glorified, states,
O you who believe! Take not the Jews and the Christians as Auliyâ’ (friends, protectors, helpers)
There are many other Ayahs (Qur'anic verses) (Part No. 1; Page No. 460) to this effect. Anyone who does this becomes on the verge of being an absolute supporter of the disbelievers, thus becoming one of them unconsciously. Consequently, he will come under Allah's, the Exalted, warning,
And if any amongst you takes them as Auliyâ’, then surely he is one of them.
...and whoever does that will never be helped by Allâh in any way, except if you indeed fear a danger from them.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.