He was old (at least in his late seventies when he became Doge), and blind, but displayed tremendous physical and mental strength. He persuaded the Crusaders to help him capture Zara in return for naval transportation to the Kingdom of Jerusalem, as well as forgivement of debts they had run up while waiting for passage from Venice.
Following the capture and sack of Zara (November 15, 1202), Alexius Angelus, son of the deposed Byzantine emperor Isaac II, arrived in that city, and with Dandolo's help, convinced the leaders of the crusade to proceed to Constantinople, with the aim of placing him on the throne of the Byzantine Empire. This led to the eventual sack of Constantinople on April 12, 1204, at which event Dandolo played a direct role.
He was active enough to take part in an expedition against the Bulgars, but died in 1205. He was buried in Hagia Sofia in Constantinople; his grave is still visible there today, though it is quite small and easy to miss.
Dandolo was from a socially and politically prominent Venetian family. His father Vitale was a close advisor of Doge Vitale II Michiel, while an uncle, also named Enrico Dandolo, was patriarch of Grado, the highest-ranking churchman in Venice. Both these men lived to be quite old, and the younger Enrico was overshadowed until he was in his sixties.
Dandolo first important political roles were during the crisis years of 1071 and 1072. In March 1071 the Byzantine government had seized the goods of thousands of Venetians living in the empire, and then imprisoned them all. Popular demand forced the doge to gather a retaliatory expedition, which however fell apart when struck by the plague early in 1072. Instead Dandolo and another ambassador were sent to Constantinople to negotiate.
During the following years Dandolo twice went as ambassador to king William II of Sicily, and then in 1183 returned to Constantinople to negotiate the restoration of the Venetian quarter in the city.
It is not known for certain when and how Dandolo became blind. The story passed around after the Fourth Crusade (and accepted by some historians) was that he had been blinded by the Byzantines during his 1171 embassy. But is is more likely that it was due to a blow to the head received sometime between 1176 and when he became doge in 1192.
Thomas F. Madden, "Venice and Constantinople in 1171 and 1172: Enrico Dandolo's Attitude towards Byzantium," Mediterranean Historical Review 8 (1993): 166-85
Thomas F. Madden, Enrico Dandolo and the Rise of Venice (Baltimore: Johns Hopkins University Press, 2003) ISBN 0-8018-7317-7
see also the references for the Fourth CrusadeReferences