The Quran

۞ ذٰلِكَ وَمَن عاقَبَ بِمِثلِ ما عوقِبَ بِهِ ثُمَّ بُغِيَ عَلَيهِ لَيَنصُرَنَّهُ اللَّهُ ۗ إِنَّ اللَّهَ لَعَفُوٌّ غَفورٌ

Muhammad Asad

Thus shall it be. And as for him who responds to aggression only to the extent of the attack levelled against him,¹ and is thereupon [again] treacherously attacked - God will most certainly succour him: for, behold, God is indeed an absolver of sins, much-forgiving.²

Lit., "who has retaliated with the like of what he had been afflicted with" - i.e., has acted only in self-defence and done to his enemy no more than the enemy had done to him. (A similar phrase, relating to retaliation in argument, is found in 16:126 and explained in the corresponding note 150.)
While the opening sentence of this verse stresses the principle of self-defence as the only justification of war (cf. 2:190 and 192-193) - with the proviso that retaliation must not exceed the injury initially suffered - the concluding part of the verse implies that in case of repeated, unprovoked aggression the believers are allowed to wage an all-out war with a view to destroying completely the enemy's military power. Since such an all-out war might seem to conflict with the principle of limited retaliation alluded to above, the Qur'an states that God absolves the believers of what otherwise might have been a sin, since it is they "against whom war is being wrongfully waged" (verse 39) by repeated acts of aggression.