وَقَٰتِلُوا۟ فِى سَبِيلِ ٱللَّهِ ٱلَّذِينَ يُقَٰتِلُونَكُمْ وَلَا تَعْتَدُوٓا۟ ۚ إِنَّ ٱللَّهَ لَا يُحِبُّ ٱلْمُعْتَدِينَ ﴿١٩٠﴾
Fight in the way of Allah those who fight you but do not transgress. Indeed. Allah does not like transgressors.
Short answer
Fighting is allowed only to defend against attack or to stop oppression, and must stop when the enemy asks for peace. It is forbidden to kill women, children, the elderly, monks and others who do not fight, and to destroy crops, trees and homes.
The fuller answer
The Prophet ﷺ instructed his armies: 'Do not betray, do not mutilate, do not kill a child.' His successor Abu Bakr told the army commander not to kill women, children and the elderly, not to cut down fruit trees, not to slaughter a sheep except for food and to leave the monks alone. Torture and burning with fire are forbidden, prisoners must be fed and treated humanely and treaties must be kept.
The Quran tells Muslims to fight only those who fight them, not to transgress limits, and to accept peace when it is offered. Muslim scholars used these rules to condemn terrorism and the killing of civilians, which are far from the rules of war in Islam. Muslims admit that their armies have not always followed these rules, but the rules are clear.
From the Quran
۞ وَإِن جَنَحُوا۟ لِلسَّلْمِ فَٱجْنَحْ لَهَا وَتَوَكَّلْ عَلَى ٱللَّهِ ۚ إِنَّهُۥ هُوَ ٱلسَّمِيعُ ٱلْعَلِيمُ ﴿٦١﴾
And if they incline to peace, then incline to it [also] and rely upon Allah. Indeed, it is He who is the Hearing, the Knowing.
لَّا يَنْهَىٰكُمُ ٱللَّهُ عَنِ ٱلَّذِينَ لَمْ يُقَٰتِلُوكُمْ فِى ٱلدِّينِ وَلَمْ يُخْرِجُوكُم مِّن دِيَٰرِكُمْ أَن تَبَرُّوهُمْ وَتُقْسِطُوٓا۟ إِلَيْهِمْ ۚ إِنَّ ٱللَّهَ يُحِبُّ ٱلْمُقْسِطِينَ ﴿٨﴾
Allah does not forbid you from those who do not fight you because of religion and do not expel you from your homes - from being righteous toward them and acting justly toward them. Indeed, Allah loves those who act justly.
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لَا تُحِلُّوا۟ شَعَٰٓئِرَ ٱللَّهِ وَلَا ٱلشَّهْرَ ٱلْحَرَامَ وَلَا ٱلْهَدْىَ وَلَا ٱلْقَلَٰٓئِدَ وَلَآ ءَآمِّينَ ٱلْبَيْتَ ٱلْحَرَامَ يَبْتَغُونَ فَضْلًۭا مِّن رَّبِّهِمْ وَرِضْوَٰنًۭا ۚ وَإِذَا حَلَلْتُمْ فَٱصْطَادُوا۟ ۚ وَلَا يَجْرِمَنَّكُمْ شَنَـَٔانُ قَوْمٍ أَن صَدُّوكُمْ عَنِ ٱلْمَسْجِدِ ٱلْحَرَامِ أَن تَعْتَدُوا۟ ۘ وَتَعَاوَنُوا۟ عَلَى ٱلْبِرِّ وَٱلتَّقْوَىٰ ۖ وَلَا تَعَاوَنُوا۟ عَلَى ٱلْإِثْمِ وَٱلْعُدْوَٰنِ ۚ وَٱتَّقُوا۟ ٱللَّهَ ۖ إِنَّ ٱللَّهَ شَدِيدُ ٱلْعِقَابِ ﴿٢﴾
O you who have believed, do not violate the rites of Allah or [the sanctity of] the sacred month or [neglect the marking of] the sacrificial animals and garlanding [them] or [violate the safety of] those coming to the Sacred House seeking bounty from their Lord and [His] approval. But when you come out of ihram, then [you may] hunt. And do not let the hatred of a people for having obstructed you from al-Masjid al-Haram lead you to transgress. And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty.
Please note
This answer follows the mainstream Sunni understanding and is general learning, not a fatwa. Where Muslims differ, the answer says so. Have a question of your own? Write to us.