Islam permitted war when it is the only way for peace. During the fight only those who are actively fighting are killed and it is forbidden to kill women, children and elderly.  Other war rules are discussed below. 

Fighting

The late scholar, al-Sayyid Sabiq wrote:

Islam pays particular attention to calling on mankind to adopt its guidance, so that they will enjoy the peaceful and blessed life of such guidance. The Muslim community is the one Allah has assigned to uphold the faith and deliver His revelations to the rest of mankind. Its task also commits it to ensure that people and communities are free. In this respect, it is the best of communities, undertaking the role of teacher. As such, it must preserve its own entity, protect its rights and strive to fulfil the role Allah has assigned to it. Any failure in this represents a great offence for which Allah administers a severe punishment of humiliation, degredation or even extinction. Islam forbids cowardice and advocacy of peace when the Muslim community has not fulfilled its task. Such peace is synonymous with cowardice and acceptance of lowly life. 

 This verse means that the Muslims have the upper hand in their faith, worship, morality, manners, knowledge and action.

Under Islam, peace is truly achieved through power. Hence, Allah has not made it absolute. He made a condition for peace, which is that the enemy should stop aggression and there be no injustice in the world so that no one will be persecuted because of his or her faith. Should a situation of this sort occur, Allah permits fighting...

Islam permits war when it is a necessity, but limits it to what is needed. No one is killed except those who are actively fighting. Anyone who stays away from it may not be killed or harmed for any reason. Moreover, Islam forbids the killing of women, children, ill people, the elderly, monks, priests and servants. Islam also forbids the disfigurement of the enemies killed in battle. Indeed, it prohibits the killing of animals and the destruction of farmland, water courses and resources and houses. Also forbidden under Islam is the killing of the wounded enemy soldiers and the chasing of those who run away. It looks at war as a surgical operation that should be confined to the diseased part of the body...

When Abu Bakr sent Usamah as Commander of an expedition to southern Syria, he gave him the following orders: Do not commit any treachery, plundering or disfigurement. Do not kill a child, an elderly, or a woman. Do not cut or burn any date tree or fruit tree. Do not slaughter any sheep, cow or camel except for food. You shall encounter peole who confine themselves in their hermitages, so leave them alone.[1]