Between 500 and 1450 , a rivalry developed between the christian europeans states and

Between 500 and 1450, a rivalry developed between the Christian Europeans States and African and Asian Muslim states. This situation evolved into the Crusades.

Crusade is a term used to refer to any of the Christian-inspired military movements from Western Europe to the Holy Land and the city of Jerusalem to conquer, occupy and keep them under Christian rule. These movements extended between the 11th and 13th centuries, when Palestine was under the control of the Sultanate of Rum. In the Middle East, the crusades were called "Frankish invasions", as local people saw these armed movements as invasions and because most of the Crusaders came from the territories of the former Carolingian Empire and called themselves Franks.  

The rich and powerful knights of the Order of St. John of Jerusalem (Hospitallers) and the Templar Knights were created during the Crusades. The term is also used, by extension, to describe uncritically any religious war or even a political or moral movement.

The term crusade was not known at the time of its occurrence. At the time, the terms "pilgrimage" and "holy war" were used. The term Crusade came about because its participants considered themselves soldiers of Christ, distinguished by the cross on their clothing. The Crusades were also a pilgrimage, a form of payment to some promise, or a way of asking for some grace, and it was considered a penance.  

By the year 1000, the pilgrimage of Christians to Jerusalem greatly increased, as there was a belief that the end of the age was near, and therefore it would be worth any sacrifice to avert hell. Incidentally, the Crusades contributed a great deal to trade with the East.

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explanation:

because the vietnam war was fought in an especially brutal and untraditional way