Question

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

Answer 1

The Venetians didn't like the idea that Constantinople was a powerful trading city. It couldn't help it -- the city was in a great spot between a number of major trading routes. So they did anything to undermine its power and the Byzantines responded in kind, sometimes by expelling their merchants.

This created a lot of bad blood between them which eventually came to a head during the Fourth Crusade. The Crusaders needed to get to Egypt but got themselves into debt with the Venetians who had the boats. In order to pay for them, they agreed to help an exiled Byzantine, Alexius IV, take the throne of the Byzantine empire, a plot supported by the Venetians since it would place the city under Latin rule as well as deal a blow against their rivals.

And so the Crusaders went off and sacked Constantinople, put Alexius IV on the throne as a Latin puppet, further splitting the Eastern and Western halves of Catholicism further apart in the Great Schism and fatally weakening the Byzantine empire even further.