The crusade wars was a defense of Christian kingdoms and of Christianity itself.
Islam spread rapidly in the middle ages from the 7th to the 10th centuries, attacking and invading territories including ones that were previously Christian such as the holy lands, the Byzantine empire in eastern Europe and the Visigoth kingdom in the Iberian peninsula western Europe, modern day Portugal and Spain.
Islam's objective was to capture all of Europe and wipe out Christianiy. By the end of the 9th century they had wiped out the Visigoth kingdom and were making advances into the Byzantine empire
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Islam was advancing into Europe from the east and from the west in the Iberian peninsula where the war against Islam became known as the reconquista. Elsewhere in Europe small kingdoms were at war with eachother and by the late 11th century they agreed to stop fighting each other and to fight the islamic invaders on these fronts before it was too late. These wars would be known as the crusades. The objective was to retake the lands that were recently Christian and halt the spread of Islam into Europe.
In the space of 200 years their would be nine crusades. Ultimately they failed in winning back the Holy land and the Bizantine empire collapsed to the Islamic Ottoman empire. However the reconquista in the Iberian peninsula was successful and the Islamic invasion into Europe was halted