After the sack of constantinople during the IV crusade (1204), Enrico Dandolo (Venetian Doge, the one who organized the thing and got the bigger share of the spoils) becomes ambitious and declare that the eastern half of the Roman Empire has undergone a Translatium Imperii, from Constantinople to Venice, such as the western half has from Rome to Acquisgrana (or wherever the german emperor lives).
The Pope is only happy to accept this and crowning him as:
1) this introduce a counterbalance to the german emperor in italy
2) get rid of orthodox doctrine having a link to the Roman Empire
3) by crowning both the wester and the eastern emperors, he gain prestige
The so-called blind emperor, however will die in just a few months (Enrico Dandolo was old and almost blind, andied in 1205)