The Christian idea or ideal of war, has no parallel in any other culture. The Christian concept is premised on 2 interlocked parts. First, it must be a 'just' war, a conflict not to engage in wanton, mindless violence; nor to indiscriminately punish and kill. The war must be fought for good reasons and higher purposes including self-defense, going to the aid of brethren being killed and oppressed, expunging an obvious evil. Second, the war must follow certain rules and be sanctioned by moral law. In this vein, war cannot slaughter the innocent, the defenceless, nor engage in 'un-Christian' acts outside the parameters of the rules of warfare. These 2 concepts – a 'just' war and a war which adheres to civilizational attitudes – can be found in Greco-Roman culture as well. Cicero, Livy and others attempted to justify the Roman wars and the attendant Roman empire – even the campaigns of Caesar [the Gaul campaign was one of imperial expansion] – by appealing to the Roman idea of a justified war. We see the same lofty, if at times impossible ideals in Christendom.
Islam is totally different. Any war to spread the Umma of Muhammad is justified. The Koran is quite clear that war must be visited upon those who oppose the cult of Muhammad. This includes Kufars [subhuman Jews, Christians, pagans], those who reject the Koran, and those Moslems who are not sufficiently Islamic enough [apostates]. War can also be justified against those 'who create mischief in the land' [5:33], or who do not submit to Islamic rule. War in other words is a central tenet of Islamic cult practice, it is the 6th pillar of Islam:
“This Christian understanding of holy war differs greatly from the Muslim teaching on jihad. Jihad is incumbent upon all Muslims and is a foundational teaching of Islam. Christian holy war is not incumbent on every believer; indeed, participation in the Crusades was always voluntary, and violence is seen as a necessary evil that can only be entered into for serious and just reasons. Christian teaching even places restrictions on the nature of warfare and on the intentions of those who participate; jihad harbors no such limitations. The main purpose of jihad is offensive through the conquering of territory in order to spread Islam throughout the world; Christian holy war is defensive and primarily involves the recovery of territory lost to an aggressor.
The huge response to Urban’s call at the council of Clermont  is even more amazing when considering the cost to go on Crusade—for a knight, four to five times his annual income.95 Although some modern critics assert that the Crusades were focused on the accumulation of wealth, the historical record shows that the vast majority of those who took the cross suffered financial hardship as result. [Weidenkopf, Glory of the Crusades, p. 19]
Christian Crusaders for example, had to pay their own way to fight to reclaim Christian land, conquered and stolen by Moslems in the Levant. Most of these men would die, and those that didn't had a good chance at going bankrupt. The Moslem Jihad was the contrary proposition. Going from a barbaric culture to conquer more civilized empires, had the opposite attraction. Booty, gold, women, arable land and estates would have been extraordinarily attractive to lure illiterate Bedouin and Arab brigands and thugs to wage war. Muhammad's entire cult was built on war and plunder and the ability to pay his men with sex and money. Without the allure of 'treasure' in the form of valuables, gold, or of female flesh which can be sex-traded, there is no Islam. Period. Thus the tenets of war in Islam are, like the rest of its doctrine, the exact opposite of Christianity.