On 8th April 1195 Isaac Angelus fell victim to a coup engineered by his elder brother Alexius, who deposed him, blinded him and seized the throne.
Alexius was an even worse Emperor than Isaac and easily manipulated. When Henry of Sicily (Holy Roman Emperor Henry VI) demanded money to pay his troops, Alexius responded by instituting a special tax and stripping jewellery from the imperial tombs. He stood by while his niece Irene, daughter of the deposed Isaac was married off by Henry to his younger brother, Philip of Swabia.
When Pope Innocent III became Pope in 1198 he proclaimed yet another Crusade. He found a leader for it in the person of Count Tibald of Champagne, who was the younger brother of Henry of Champagne, who had ruled the Kingdom of Jerusalem from 1192 until 1197. It was decided that Egypt was the prime target. This meant that the Crusaders would need to travel by sea. Consequently a party of knights led by Geoffrey de Villehardouin, Marshal of Champagne, arrived in Venice to acquire transport. The eighty year old blind Doge of Venice, Enrico Dandolo, agreed to provide it, but at a cost of 84,000 silver marks and half the territories conquered.
When the Crusaders met a year later on 24th June 1202, they found that their numbers were only a third of what they had expected and with not enough money to pay the Venetians. After Dandalo had extracted as much as he could from them, he agreed to forgo the rest if the Crusaders would re-capture the Venetian city of Zara which had recently fallen to the Hungarians.
On 8th November 1202 the Crusaders set sail with Dandalo on board, but not to the Holy Land, but to Zara. When the Pope found out he excommunicated the lot of them. Early the next year, a proposal arrived from Philip of Swabia, brother of the Holy Roman Emperor, Henry IV, and son in law of the deposed Isaac II. Isaac's son, Alexius, had escaped from prison and found refuge with Philip. If the Crusade would depose Alexius III and replace him with the younger Alexius, Philip would finance the subsequent Crusade to the Holy Land. Dandalo loved the idea and the Crusaders did as well. On 24th June 1203, the Venetian ships dropped anchor off Constantinople.
Alexius III had made no preparations for the city's defence. The Crusaders were able to land below Galata, on the north-eastern side of the Golden Horn and take the single tower there. The chain that blocked the entrance to the Golden Horn was lowered and the Venetian fleet sailed in. The city walls were then attacked from land and sea, with Doge Dandalo standing in the prow of his galley inciting his men to fight. Soon the Crusaders were pouring into the city, setting fire to the wooden buildings. That evening Alexius III fled.
The Byzantines rushed to get the old, blind Isaac II out of prison and restored to the throne. This should have removed the grounds for intervention by the Crusaders, but the latter would not leave until the young Alexius was installed as co-emperor. On 1st August 1203, Alexius IV Angelus was crowned alongside his father.
Alexius had promised the Crusaders money. He made himself unpopular by melting down church plate, but was still unable to come up with the cash. The Crusaders, now under the direct leadership of Dandalo, resolved to retake the city. In January 1204, Alexius Ducas - nicknamed Murzuphlus because of his eyebrows, which met in the middle - now seized the throne, killing Alexius and Isaac. Murzuphlus was crowned in St Sophia as Alexius V, 5th February 1204. On 12th April, the Crusaders took Constantinople, and Murzuphlus fled. Byzantium was to be ruled by Latin princes for the next 57 years.