أُذِنَ لِلَّذِينَ يُقَاتَلُونَ بِأَنَّهُمْ ظُلِمُوا ۚ وَإِنَّ اللَّهَ عَلَىٰ نَصْرِهِمْ لَقَدِيرٌ
“Permission [to fight] has been given to those who are being fought, because they were wronged. And indeed, Allah is competent to give them victory.”
This is the exact ayah the event's own description quotes -- the first verse permitting military jihad, after thirteen years of commanded patience.