Why did God permit the believers to fight after migration from Mecca to Medina?

Verses 39-40 of Chapter 22:

Permission is granted to those against whom the disbelievers wage war, as they are being wronged; and indeed Allah is Able to assist them. Those who were unjustly expelled from their homes just because they said, "Allah is Our Lord" and had Allah not repelled some men by means of other men, the abbeys, churches, synagogues and mosques - in which the name of Allah is profusely mentioned - would definitely be demolished; and indeed Allah will assist the one who helps His religion; indeed surely Allah is Almighty, Dominant.