When the Third Crusade failed to regain Jerusalem from the Ayyubids, Pope Innocent immediately started to encourage a new crusade. It took a long time until the crusade took place for a variety of reason. In 1193 AD, Saladin had died. As a cause of this the crusaders thought that Saladin's successors were weaker and would be easier to beat.
For this crusade the crusaders would try something different. Rather than coming down from the north, they would sail the opposite direction to Egypt. Then they would come up from Egypt to Jerusalem.
The Crusaders did not have enough ships to take everyone to Egypt. So they went to Venice, the great sea power. When the Crusaders went to Venice, in June 1202, they did not have enough money for the ships. So the Venetians made a deal with the Crusaders. The Venetians agreed to finish the needed ship for the crusade if the Crusaders would first seize Zara on the eastern coast of the Adriatic. Zara, however, was a Christian city. Therefore the Pope protested against this. Despite this the crusaders besieged and captured the city.
"The interest of the Venetians and the intention of Dandolo, of course, was not merely to take Zara, but to secure it under their own hegemony." (Book Internet)
After the seize of Zara the Venetians persuaded the Crusaders to turn their arms against Constantinople. For the Venetians, Constantinople would greatly increase trade and influence in the East; and for the crusading nobles it gave opportunities for the growth of wealth and power.
The Crusaders took Constantinople. They burned a great part of it and slaughtered the inhabitants. They also destroyed monuments, statues, paintings, and manuscripts; which took thousands of years of accumulate.
Many islands which had belonged to the Empire now belonged the Venetians. At the end of the Fourth Crusade the Crusaders never fought the Ayyubids at all, and never went to Jerusalem. At the end, they took the piles of money and jewels and gold that they had captured in the sack of Constantinople and they went home. Pope Innocent agreed to let them back into the Church.
The chief crusaders formed part of the remaining territory into the Latin Empire of Constantinople. It was organized in fiefs. This new Empire lasted for less than sixty year. At the end of this period the Greeks returned to power. "The fall of Constantinople to the Venetians and the soldiers of the fourth crusade in April 1204 was its climax." (Book Internet)