It was narrated that Ibn "Abbas said:

When the Prophet (ﷺ) was expelled from Makkah, Abu Bakr said: They have driven out their Prophet (ﷺ); verily to Allah we belong and unto Him is our return, they will certainly be destroyed. Then the verse “Permission to fight (against disbelievers) is given to those (believers) who are fought against, because they have been wronged; and surely, Allah is Able to give them (believers) victory" [al-Hajj 22:39] was revealed. Then he realised that there would be fighting. Ibn "Abbas said: This was the first verse to be revealed about fighting.