The Entry of the Crusaders into Constantinople (Eugène Delacroix, 1840)
Jonah Goldberg has a simple view of the Crusades:
Historian Thomas Madden puts it more directly, “Now put this down in your notebook, because it will be on the test: The crusades were in every way a defensive war. They were the West’s belated response to the Muslim conquest of fully two-thirds of the Christian world.”and
The truth is that the crusades had nothing to do with colonialism or unprovoked aggression. They were a desperate and largely unsuccessful attempt to defend against a powerful enemyand
The Crusades were launched not as a war of conquest but as a war to save Christians from Muslim persecution and conquest.
Jonathan Jarrett rightly thinks it's more complicated.
We can guess at the motives of a few crusaders.7 Despite the emphasis laid on his piety and good conduct by the author of the Gesta Francorum,8 the actions of Bohemund on crusade, as well as his previous and subsequent ventures against the Byzantine Empire, make it seem plausible that he at least, a prince without a principality, was out to grab land and the fame and money necessary to keep it.9 On the other hand, Professor Riley-Smith among others has drawn attention to Raymond of Saint-Gilles’s membership of the body known as the fideles beati Petri to suggest that his motives might be most easily seen in terms of personal devotion to the ideals of Pope Urban, not just devotional then but ecclesiological,10 and it is difficult to imagine why he should have abandoned his position as one of the richest and most powerful men in France on such a risky venture if not principally for spiritual reasons.
Bohemund seems to have been a slippery and opportunistic adventurer. Antioch had been held by the Byzantines as late as 1078. From 1080-85 Bohemund and his family were invading the Byzantine Empire, penetrating into Thessaly before being defeated and forced to retreat. In Byzantium in 1097 he swore that he would return all former Byzantine possessions to the emperor, but he seized Antioch as a principality for himself in 1098. Soon after the principality was trying to seize Byzantine controlled territory to its north.
After the crusaders lost the Battle of Harran in 1104, Antioch lost territory to the Turks. Bohemund returned to Europe to raise a new army, but instead of using it to recover Christian losses in the Levant he used it to unsuccessfully invade the Adriatic coast of the Byzantine Empire in 1107.
Bohemund's primary goal seems to have been to conquer territory overseas where he could settle and rule the natives for his own benefit. The religion of the current ruler of those lands was apparently not a a major factor in his decisions. In his case, colonialist wars of conquest seem like a fair description of his goals.
So even parts of the crusader effort in the Levant contradict Goldberg's description. And this was only part of what Christendom considered crusades. In Iberia, a Christian counter-offensive against Muslims was ultimately entirely successful, and ended by conquering people that had been ruled by Muslims longer than by Christians.
Starting in 1147 a series of crusades were launched not against Muslims, but against pagans around the eastern and southern coasts of the Baltic and later against Orthodox (and Christian) Russia.
The Fourth Crusade was originally targeted against Muslim forces that controlled much of the Levant, but ended with the sack and dismemberment of the Christian Byzantine Empire without ever reaching the Holy Land.
Other crusades were launched against heretics, people who were insufficiently zealous in suppressing heretics, people who fought vassals of the current pope, people that supported the wrong pope and people that supported the right pope but were allied with people that supported the wrong pope.