The First Crusade
The First Crusade originated in Rome in 1094, when Pope Urban II declared that the Catholic states of Europe, mainly England, France, and the Holy Roman Empire, to declare a crusade against the Orthodox Christian Byzantines and their rapidly expanding Muslim community, and declared the citizens of the Byzantine Empire heretics. The Pope spent two years campaigning for troops, until he had raised an army of 35,000 soldiers, 30,000 infantry and 5,000 cavalry, to invade the Byzantine Empire. The Pope's plan was to send the army of Crusaders by sea to capture the Holy Land, before moving to capture Constantinople.
The Byzantines were taken by surprise in 1106, when the Crusaders landed at Acre, and captured the city in two days. The Byzantine Emperor declared war on the Crusaders, and sent a Byzantine army of 50,000 to expel Crusaders from Acre. As the Crusaders marched south, They captured Jerusalem in early 1097, and were then set into battle with the Byzantine Army, ordered to destroy the Crusader armies, an were pushed back after a four-hour battle. The failure to recapture Jerusalem was a great disgrace to the Byzantine Army, who were depleted in troops, and fought only small skirmishes against the Crusaders.
The Crusaders continued to capture more Byzantine land in Judea, and the city of Ashkelon fell in 1098, marking an end to Byzantine resistance to the Crusaders. The rest of the war was by the Crusaders in defense from multiple Byzantine counterattacks, which ceased in early 1099. The Pope was finally sent a letter from Emperor Alexios, asking for a peace. The Pope accepted, and the result was the creation of the Church-owned Kingdom of Jerusalem, and a national disgrace of the Byzantine Empire.
The Second Crusade
In 1145, tensions between the Byzantine Empire and the Kingdom of Jerusalem had escalated to the point that the Byzantine declared war and launched an invasion of the Kingdom of Jerusalem. The Kingdom of Jerusalem asked for help from the Pope, the form of a Second Crusade, which the Pope agreed to, and then subsequently sent an army of 45,000 to Jerusalem to counter the Byzantine invasion. The Byzantines captured Acre in late 1145, and then continued down to capture much of the coast.
The Crusaders and Byzantine first met outside of Jerusalem, where the Byzantines won the battle of the Holfy Fields outside the city of Jerusalem, Byzantium now had reconquered Jerusalem. The Crusaders then set to campaign in the north of Judea, where they then proceeded to launch a highly successful, climaxing with the capture of Antioch in 1147. The Byzantines then went to campaign in the north, were they were met by by a Crusader army with very high morale. The two armies met at Antioch in early 1148, where the battle ended in a stalemate, and the two sides decided to end the war there.
The two sides signed the Treaty of Antioch in 1149, which ended the Second Crusade in a status quo ante bellum.
The Third Crusade
The Third Crusade began when the Byzantine Emperor, Isaac II, wanted to finish of the Crusaders once and for all, and declared war on the Kingdom of Jerusalem in 1189. The Pope responded by sending an army of 65,000 soldiers to Jerusalem to fight the Byzantines, who were invading Jerusalem with an army of 150,000 soldiers. The Byzantine captured the city of Antioch, then Tripoli, and then Damascus, to ends it campaign in 1190.
The Crusaders responded with a campaign to capture the Byzantine province of Aegyptus, where they achieved limited success, and captured the city of Alexandria, but failed in their attempt to capture the city of Cairo. The Byzantine finally captured the city of Jerusalem in 1190, and then finished capturing the Kingdom of Jerusalem by early 1191, and moved to recapture the recently declared Holy State of Alexandria. The Byzantines moved to cut off the city by land and sea, and then proceeded to lay siege to the city.
The siege lasted four months, and then in 1192 the city's defenders surrendered, and the city was now back in Byzantine hands, and so were Richard I, Phillip II and Frederick I. They were each handed back to their respective countries in exchange for a ransom, and the Pope then asked the Byzantine Emperor for peace.
Treaty of Rome
The Treaty of Rome was signed in St. Peter's Basilica in Rome in 1193, between Pope Celestine III and Byzantine Emperor Alexios I. The Treaty officially disestablished the Kingdom of Jerusalem and the Holy State of Alexandria, and the Pope agreed to a ten-year peace agreement, although this would eventually be followed by the Eternal Peace of Constantinople.