On Tuesday, November 27, 1095, Pope Urban II launched a series of battles known as the Crusades. The goal was to launch a counterattack against the Seljuks in Anatolia then defeat the Muslims in Syria and Palestine and eventually retake Jerusalem from the Egyptians. Most of the crusaders were nobles. By 1096, five major armies of noblemen--mainly French--had been assembled and set out on their journey.
The common people also got caught up in the excitement and organized their own "popular Crusade" led by a preacher known as Peter the Hermit. Very few of the popular Crusaders made it to the Middle East and even less lasted until Jerusalem.
This First Crusade was successful in its holy war, slaughtering Muslims everywhere they went and taking many cities. But after these attacks, the Muslims became more unified and organized under Imad ad-Din Sangi, ruler of Al Mawsil and Halab (Aleppo). They fought back and retook the city of Edessa in 1144.
So in 1145 the Second Crusade was launched. These Crusaders fell into ambushes and lost battles and basically failed. The Muslims regrouped again and began retaking more territory from the Crusaders. In 1187, they took back Jerusalem, leaving Typre in Lebanon as the only major city still occupied by the Crusaders.
The Third Crusade didn't work very well either. Although they had lots of people, all they got was a chain of cities along the Mediterranean. From there, the Crusades basically went down hill. The fourth one, in 1202-1204, had financial difficulties. To get money, they took Constantinople and plundered it. In 1217, the Fifth Crusade was launched. The plan was to attack Egypt, take Cairo, and then get control of the Sinai Peninsula. But promised reinforcements didn't show and the campaign failed.
Holy Roman Emperor Frederick II kept promising to lead his own Crusade but postponing it because of political stuff at home. Finally, in 1227, he set off but returned in a few days because he got sick. The Pope was not a happy camper and excommunicated him. Frederick decided to go again in 1228, even though he was excommunicated. His Crusade was unique, consisting almost completely of diplomatic negotiations with the Egyptian sultan Al-Kamil. Frederick got a peace treaty giving Jerusalem to the Crusaders and guaranteeing a 10-year break from fighting. But the Pope still didn't like him and proclaimed a Crusade against Frederick, got an army and attacked his Italian possessions.
Twenty years later the next big Crusade to the Middle East was launched by King Louis IX of France when the Muslims took back Jerusalem in 1244. After four years of planning, he headed to Egypt. After capturing the port of Damietta, they attacked Cairo. The Crusaders hadn't guarded their flanks and the Egyptians kept control of the water reservoirs by the Nile. They opened the gate and trapped the whole Crusading army in floods. The Crusaders had to pay a huge ransom and give back Damietta before returning home.
The last major Crusade came in 1270, again organized by King Louis (he apparently liked being humiliated). The French nobles weren't particularly enthusiastic. Instead of Egypt, Louis decided to attack the city of Tunis. But the Crusade ended quickly when Louis died in the summer of 1270.