This Medinan verse was the first revealed to be about the Jihad:
أُذِنَ لِلَّذِينَ يُقَاتَلُونَ بِأَنَّهُمْ ظُلِمُوا ۚ وَإِنَّ اللَّـهَ عَلَىٰ نَصْرِهِمْ لَقَدِيرٌ (٣٩) To those against whom war is made, permission is given (to fight), because they are wronged; and verily, Allah is most powerful for their aid. (Qur'an 22:39)
Before this verse was revealed, the companons asked the Prophet for permission for fighting to allow them to fight, but the Prophet didn't.