أُذِنَ لِلَّذِينَ يُقَٰتَلُونَ بِأَنَّهُمۡ ظُلِمُواْۚ وَإِنَّ ٱللَّهَ عَلَىٰ نَصۡرِهِمۡ لَقَدِيرٌ ٣٩
Permission [to fight] has been given to those who are being fought,1 because they were wronged. And indeed, Allāh is competent to give them victory.
Translation: Saheeh International (English)
1 Referring here to the Prophet's companions.
Each word's meaning and sound
Allah has given permission to fight to the believers who are fought by the idolaters, due to their being oppressed by their enemies. Allah has the power to help the believers against their enemies without fighting. However, the wisdom of Allah required that the believers be tested through fighting the disbelievers.
Al-Mukhtasar in interpreting the Noble Quran