When critics of Christianity list the sins committed by Christians throughout history, invariably they will mention the Crusades to the recover Jerusalem from the Muslims and the conquest of Muslim lands in the Levant from the 11th through the 13th centuries.
Some Christian apologists will counter that the Crusades to retake the "Holy Land", such as Robert Spencer in his book The Politically Incorrect Guide to Islam (and the Crusades), were in actuality defensive conflicts. The reasoning employed by Spencer and others is that the Crusades were a counter-offensive to recover land that was taken by the Muslims from the Christians during the 7th century.
This reasoning fails on a number of levels. If one wants to be technical about it, the lands of Palestine, Syria, and Egypt were provinces of the Eastern Roman (or Byzantine) Empire before they were conquered by the Arab Muslims. The reason why the Muslims were able to take Jerusalem and the "Holy Land" is because the Byzantines and the Persians had beat each other to a bloody pulp like two heavyweight boxers only a few years earlier and were too weakened to resist. Their treasuries depleted, neither of the two regional superpowers of the day were able to continue paying their respective Arab client states, so they threw in their lots with their kindred Arabs. Thus, the Byzantines and the Persians lost their valuable buffer states. For the Byzantines, the situation was further aggravated due to the fact that their attempts to impose their brand of orthodoxy on their fellow Christians in the Near East were not welcomed by the latter. The Muslims, on the other hand, did not care what kind of Christianity their subjects practiced, as long as they paid their taxes.
If the Crusades were really a counter-offensive, then ideally the lands recovered by the Crusaders should have been returned to their previous Christian rulers, the Byzantines. Problem was, the Byzantines were Eastern Orthodox, whereas the Crusaders were Catholics whose religious allegiance was to the pope in Rome. And the two churches had formally split in the Great Schism of 1054.
The truth is, the Crusades to recover and hold Jerusalem were not about fighting a Muslim threat. Instead, they were carried out for entirely different purposes. For the popes, the Crusades were a great way to get the Christian powers of Western Europe to stop fighting each other and to instead channel their aggression to fight a far off enemy that posed no threat to them. The popes also had in mind to expand their hold over Christendom by forcing the militarily weakened Byzantines to accept the supremacy of the Catholic Church.
If the Crusaders were really serious about engaging in a counter-offensive against the Muslims, the logical first step would have been to assist the Byzantines in expelling the Muslims from Asia Minor. In fact, it was the Byzantine request for aid to recover territory in Asia Minor from the Seljuk Turks that served as the catalyst for the Crusades. As long as the Anatolian heartland remained in Muslim hands, the security of the Crusaders states in the Levant could not be maintained for long. While relations between the Byzantines and the Crusaders were frosty at best, the humiliating Byzantine defeat at Myriokephalon in 1176 confirmed the permanent presence of Islam in Asia Minor and deepened the isolation of the Crusader states. So much then for the Crusades being a counter-offensive against Muslim aggression.