Question

What role did merchants from Venice, Italy play in the decline of the Byzantine Empire?

Answer 1

The short explanation is that the Byzantines had, at an earlier point in their history, had expelled Venetians and confiscated their properties within Constantinople in response to their belligerence. Later, a massacre of Latin speakers (who were also Roman Catholic) erupted unchecked throughout the city, further worsening relations.

This created a lot of bad blood between East and West. When the Fourth Crusade came around and the armies needed transport to Egypt to fight for the Holy Land, the fleet was supplied by Venice in exchange for one or two "side trips" to help pay back the loan. The last destination was Constantinople which the Venetians, encouraged by Enrico Dandolo (who suffered at the hands of the Byzantines many years earlier), convinced the Crusaders to sack under the premise of overthrowing the current emperor and installing a new one.

The sack of Constantinople dealt a humiliating blow to the already weakening Byzantine empire that it never really recovered from. Although the city was eventually taken back, things were never the same between East and West and the Byzantine Empire continued to decay.