Muslim incursions in North Africa were not the reason for the Crusades. North Africa was conquered by Muslims over 300 years before the Crusades started.

The causes of the crusades vary but the main trigger of them occurred when the Byzantine emperor asked for help from the Pope to hold back the Muslim Turks who had invaded Anatolia (modern Turkey) after the Battle of Manzikert in 1071 and set up kingdoms of their own throughout Anatolia. The Pope then called for a crusade throughout Christian Europe which initiated the crusades as many European Christians went East to conquer the Holy land. Other factors leading the crusades included tensions between the Seljuk and the Fatimids who fought mainly in the Holy land which disrupted Christian pilgrimages which made many Europeans unhappy.

This is just a small picture into the causes of the Crusades and by no means paints an entire picture of the political situation of the period. There were many other factors that lead to the Crusades some of which were caused by Muslim aggression but many others which weren't.

EDIT: Since I've been asked to give a few examples of Christians causing the Crusades I'll give a couple. Christian Europe was experiencing a period of internal turmoil during this period with many warlike groups throughout the area. Groups such as the Normans were very warlike and were really looking to fight almost anyone (this can be seen through the Norman conquests of England and Southern Italy. Groups such as this were really just looking for a fight and the Call to arms by the Pope really just gave them another target. Some scholars even argue that the Pope intentionally called the Crusade in the hope that many of these warlike people would leave Europe and allow for a more peaceful existing (I read one source that went as far as to suggest that the Pope wanted to turn Europe into a Theocracy and that he wanted these warlike people to leave as they were the most likely to fight him but from what I understand that is not the common opinion on the Pope's desires). Another major factor that lead to the Crusades was the shift to primogeniture succession in Europe (the first-born inherits everything) that lead to many powerful, landless nobles in Christendom. Many of these landless nobles joined the Crusade in order to gain land and titles out East (this would also be a common trend among Spanish conquistadors and the conquest of the New World over 400 years later).

Hi, this sounds like a variation of the "the Crusades were a defensive action" claim that swirled around this sub last February. Here are a couple of posts that attempt to unpack this idea

• What Caused the Crusades? - /u/thejukeboxhero responds to a slightly different question, but ends up addressing this "Muslim aggression" idea. The response includes links to several other, more directly on-topic posts, all of which are highly recommended. And don't miss the last link, a post from the FAQ with a fantastic overview of the Crusades by /u/ambarenya

• Were the Crusades a defensive move? - more from /u/thejukeboxhero

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