My very first campaign as Venice, I did the council missions and hit the rebels in the immediate east hard. However, I paused and only started building troops and attacking the Byzantines after they attacked me first. Needless to say, I was unprepared when Milan double crossed me so I restarted after they were on the verge of capturing Venice. I think the HRE attacked me as well and the Hungarians were close to attacking.

I restarted and hit the rebels and then the Byzantines immediately. After conquering Thessalonica, I slowly conquered the Byzantines and was able to defend Venice.