Islam does not teach aggression as did Judaism through the law of retaliation. Nor does it, like present-day degraded Christianity, preach a contradiction. It does not ask us to turn the other cheek and at the same time to sell our clothes to buy a sword.
The teaching of Islam fits into the natural instincts of man, and promotes peace in the only possible way. Indeed, Islam preaches the middle way, neither too extreme, nor too lenient, but justice should be justly done according to the situation prevailing, but like Allah advises in the Holy Quran, forgiveness and patience are best especially in respect to killings and aggression.
All in all, Islam forbids aggression, but it urges us to fight if failure to fight jeopardises peace and promotes war. If failure to fight means the extirpation of free belief and of the search of truth, it is our duty to fight. This is the teaching on which peace can ultimately be built, and this is the teaching on which the Holy Prophet of Islam, Hazrat Muhammad (pbuh) based his own policies and his practice.