Why did God not allow fighting to take place at Hudaibiyah?

Verse 25 of Chapter 48:

They are those who disbelieved and hindered you from Al-Masjid Al-Haram and detained the sacrificial animals, from reaching their place of sacrifice. Had there not been believing men and believing women whom you did not know, that you may kill and on whose account a sin would have been committed by you without (your) knowledge, that Allah might bring into His mercy whom He wills -- if they had been apart, We verily, would have punished with painful torment those of them who disbelieved.