The Fourth Crusade
In 1200 AD, Pope Innocent began to ask the rulers of Europe to participate in a fourth crusade, again attempting to take Jerusalem away from the Ayyubids who ruled there. Saladin had died in 1193 AD, and the Crusaders thought his successors were weaker and would be easier to beat. This time they would try something different. Instead of coming down from the north, the European armies would sail south to Egypt, and then come up from there to Jerusalem.
In order to get enough ships to take everyone to Egypt, the armies needed help from the great sea power, Venice. In 1202, the Crusaders came to Venice to get their ships, but they didn't have enough money to pay for them. So the Venetians said, "Okay, you can pay us later, but in exchange you have to fight for us to get back the city of Zara (in modern Hungary) that went over to the Hungarians a few years ago." The Crusaders agreed to do this, even though Zara was a Christian city. The Pope didn't like this and excommunicated all the Crusaders.
Crusaders get to Constantinople
The Crusaders succeeded in taking Zara, and were about to go on to Egypt with their ships when Alexius Comnenus, who had recently been thrown out of Constantinople, asked the Crusaders to help him get into power again. He said he would pay for the rest of the Crusade, once he was back on his throne. Instead of going to Egypt, the Crusaders agreed to this plan, and in 1203 (with the help of the Venetians) they took Constantinople and put Alexius IV on the throne. But Alexius IV could not raise the money he had promised, and when he tried to raise the money through taxes he became so unpopular that he and his father were killed and a new emperor, Alexius V, got on the throne.
Constantinople - 1204 AD
In 1204 the Crusaders and Venetians attacked Constantinople and sacked the city. A lot of the islands which had belonged to the Empire were taken over by the Venetians too. The Crusaders never did go on to Jerusalem, and never fought the Ayyubids at all. They took the piles of money and jewels and gold that they had captured in the sack of Constantinople and they went home. The Pope agreed to let them back into the Church.