I find it strange that people and historians are so fixated on the 4th Crusade as the be all end all of the Byzantine's and ignore or minimize as much as possible all that irrelevant side business of the centuries of fighting with the Muslims. You know, the guys that fought them far more often and more brutally and actually took away nearly all their territory and destroyed them? In addition to the above comment and a few others in this thread take this video for example.