The Quran is unequivocal and mubeen (perfectly clear) on the subject of war: it is prohibited to be fought as a means of converting people to Islam——
لَا إِكْرَاهَ فِي الدِّينِ قَد تَّبَيَّنَ الرُّشْدُ مِنَ الْغَيِّ فَمَن يَكْفُرْ بِالطَّاغُوتِ وَيُؤْمِن بِاللَّهِ فَقَدِ اسْتَمْسَكَ بِالْعُرْوَةِ الْوُثْقَىٰ لَا انفِصَامَ لَهَا وَاللَّهُ سَمِيعٌ عَلِيمٌ
(Surat Albaqara 2:256)
There is no compulsion in religion; the proper way has been clearly distinguished from the wrong way. Whoever rejects evil, and believes in Allah, indeed he has taken grasp of the strongest hold that will never break. Allah is Hearer, Knower.
Islam, according to the Quran, does not permit the acceptance and adoption of a belief system to be by force. All true belief must be freely chosen from the heart. Free will is an essential component of faith. Allah prohibits forcing people to convert to a particular religion or to adopt a belief system. To force others to convert to Islam (or any belief, for that matter) is oppression, and hence incurs Allah’s wrath. Contrary to what is being promulgated by some, the above aya unequivocally states that such use of force is both “wrong” and “evil”.