The First Crusade (1095 - 1099), which lasted only four years following the initial declaration by Pope Urban, was amazingly successful. The Abbasid Caliphate had long since splintered apart, with rival kingdoms holding power in North Africa and the Middle Ages. The doctrinal differences between Sunni and Shia Muslims further divided the Muslim Ummah. In addition, the Arab kingdoms battled the Seljuk Turks, who were intent on conquering everything, not just Christian lands. Thus, the Crusaders arrived precisely when the Muslim forces were profoundly divided. By 1099, the Crusaders had captured Jerusalem and much of the Levant, forming a series of Christian territories in the heart of the Holy Land. These were called The Latin Principalities, kingdoms ruled by European knights.
After their success in taking Jerusalem, the knightly orders became very powerful and very rich. They not only seized loot, but became caravan guards and, ultimately, money-lenders (the Templars became bankers after abandoning the Holy Land when Jerusalem was lost in 1187). Essentially, the major orders came to resemble armed merchant houses as much as monasteries, and there is no question that many of their members did a very poor job of living up to their vows of poverty, obedience, and chastity. Likewise, the rulers of the Latin Principalities made little effort to win over their Muslim and Jewish subjects, treating them instead as sources of wealth, infidels unworthy of humane treatment.
Subsequent Crusades were much less successful. The problem was that, once they had formed their territories, the westerners had to hold on to them with little but a series of strong forts up and down the coast. The European population centers were obviously hundreds or thousands of miles away and the local people were mostly Jews and Muslims who detested the cruel invaders.
Attacks on the Latin Principalities resulted in the Second Crusade, which lasted from 1147 - 1149. The Second Crusade consisted of two Crusades that happened simultaneously: some European knights sailed off to the Holy Land, while others fought against the Cordoban Caliphate in the Iberian Peninsula. The Europeans ultimately lost ground in the Middle East but managed to retake Lisbon in Portugal from the Muslim Caliphate there. In fact, the Second Crusade’s significance is that crusaders began to wage an almost ceaseless war against the Cordoban Caliphate in Spain - in a sense, Christian Europeans, particularly the inhabitants of the Christian kingdoms of northern Spain, concluded that there were plenty of infidels much closer to home than Jerusalem and its environs. These wars of Christians against Spanish Muslims were called the Spanish "Reconquest" (Reconquista), and they lasted until the last Muslim kingdom fell in 1492 CE.
In 1187 an Egyptian Muslim general named Salah-ad-Din (his name is normally anglicized as Saladin) retook Jerusalem after crushing the crusaders at the Battle of Hattin. This prompted the Third Crusade (1189 - 1192), a massive invasion led by the emperor of the Holy Roman Empire (Frederick Barbarossa), the king of France (Philip II), and the king of England (Richard I - known as "The Lion Heart"). It completely failed, with the English king negotiating a peace deal with Saladin after Frederick died (he drowned trying to cross a river) and Philip returned to France. After this, only a few small territories remained in Christian hands.
Arguably the most disastrous (in terms of failing to achieve its stated goal of controlling the Holy Land) crusade was the Fourth Crusade, lasting from 1199 – 1204. This latest attempt to seize Jerusalem began with a large group of crusaders chartering passage with Venetian sailors, long since accustomed to profiting from crusader traffic. En route, the crusaders and sailors learned of a succession dispute in Constantinople and decided to intervene. The intervention turned into an outright invasion, with the crusaders carrying out a horrendously bloody sack of the ancient city. In the end, the crusaders set up a Latin Christian government that lasted for about fifty years while completely ignoring their original goal of sailing to the Holy Land. The only lasting effect of the Fourth Crusade was the further weakening of Byzantium in the face of Turkish invaders in the future. To emphasize the point: Christian knights from Western Europe set out to attack the Muslim kingdoms of the Middle East but ended up conquering a Christian kingdom, and the last political remnant of the Roman Empire at that, instead.
Many further crusades followed; popes would continue to authorize official large-scale invasions of the Middle East until the end of the thirteenth century, and the efforts of Christian knights in Spain during the Reconquest very much carried on the crusading tradition for centuries. Later crusades were often nothing more than politically-motivated power grabs on the part of popes, launched against a given pope’s political opponents (i.e. fellow European Christians who happened to be at odds with a pope). Technically, the last crusade was the Holy League, an army drawn from various kingdoms in Central and Eastern Europe dispatched to fight the Ottoman Empire in 1684. None of the latter crusades succeeded in seizing land in the Middle East, but they did inspire a relentless drive to overthrow and destroy the now centuries-old Muslim kingdom of Spain, as noted above, and they also inspired the idea of the potential “holiness” of warfare itself among Christians.