Who’s given permission to fight in the way of Allah?
Permission (to fight) is given unto those upon whom war is made for they have been oppressed, and verily, to help them, God is Most Potent; those who have been expelled from their homes unjustly save that they say: Our Lord is God!?
The Holy Qur'an (Surah22, The Hajj, verse 39-40).
Other links:
Qur'an as a guide
