<P> The Kingdom of Jerusalem was the first experiment in European colonialism creating a' Europe Overseas' or Outremer . The raising, transportation, and supply of large armies led to flourishing trade between Europe and the outremer . The Italian city states of Genoa and Venice flourished, creating profitable trading colonies in the Eastern Mediterranean . This trade was sustained through the middle Byzantine and Ottoman eras, and the communities were often assimilated and known as Levantines or Franco - Levantines . </P> <P> The Crusades consolidated the papal leadership of the Latin Church, reinforcing the link between Western Christendom, feudalism, and militarism and increased the tolerance of the clergy to violence . The growth of the system of indulgences became a catalyst for the Protestant Reformation in the early 16th century . The Crusades also had a role in the creation and institutionalisation of the military and the Dominican orders as well as the Medieval Inquisition . </P> <P> The behaviour of the Crusaders appalled the Greeks and Muslims, creating a lasting barrier between the Latin world and both the Islamic and Orthodox religions . It was an obstacle to the reunification of the Christian church and created a perception of Westerners as defeated aggressors . </P> <P> Many historians argue that the interaction between the western Christian and Islamic cultures was a significant, ultimately positive, factor in the development of European civilisation and the Renaissance . The many interactions between Europeans and the Islamic world across the entire length of the Mediterranean Sea led to improved perceptions of Islamic culture, but also make it difficult for historians to identify the specific source of various instances of cultural cross-fertilisation . The art and architecture of the Outremer show clear evidence of cultural fusion but it is difficult to track illumination of manuscripts and castle design back to their sources . Textual sources are simpler, and translations made in Antioch are notable but considered secondary in importance to the works emanating from Muslim Spain and the hybrid culture of Sicily . In addition, Muslim libraries contained classical Greek and Roman texts that allowed Europe to rediscover pre-Christian philosophy, science and medicine . </P>

Who were the crusades and why did they fight