The Crusades

"If the Muslim right to Jerusalem was solely by force of conquest, why wouldn't the Crusaders have the right to take it back by force?" ~ Dinesh D'Souza

Right. Although, actually, the Jerusalem has surrendered to the invading Crusaders without the fight, however the next morning all hell broke loose: 

"The next morning, however, Jerusalem became a killing field as the conquerors {Crusaders) slaughtered nearly every Muslim in the city and burned down a synagogue in which Jews had sought refuge. “With drawn swords our men ran through the city not sparing anyone, even those begging for mercy,” wrote Fulcher of Chartres, who served as Baldwin’s chaplain. “They desired that this place, so long contaminated by the superstition of the pagan inhabitants, should be cleansed from their contagion.”"*1

By the end of the ensued slaughter over 70,000 inhabitants were killed. The Jerusalem was cleared of its native residents to such an extent that for years after the official language of the city was French due to many Crusaders speaking it.

Although there were numerous Muslim conquests throughout the ages, however much of these Muslim invasions is rather later time propaganda, describing in simple terms the complex historical events that took place, and the same can be said about the Crusades. 

The crisis first manifested with the advance of the Muslim armies that threatened and eventually closed the ancient trade routs from Europe to the Holy Land, which the knights kept safe for the pilgrims. Afterwards, the Pope Urban II has called for the Holy War and sent the restless European bullies to fight the religious wars in the Middle East instead of terrorizing the local population and businesses, because the Templar Knights, as they later became known, had a special arrangement with the Pope to have the right to "rent" the money, since the usury was not allowed to practice by neither Christians nor Muslims.

That caused the Templar Knights to become very wealthy and powerful, so and so, that they were seen as a threat to the status quo. Thus the Pope, on one hand, managed to get rid of the annoying knights from Europe, and on the other, attempted to reopen the pilgrim and trade routes which were crucial for the Christian world and its economy.

*1 -  http://www.biblicalarchaeology.org/daily/biblical-sites-places/jerusalem/what-were-the-crusades-and-how-did-they-impact-jerusalem/