[festus]

Rome fell because it didn’t kill the anti-Romans faster than the anti-Romans killed the Romans.

[muawiyah]

The Western Empire fell when the illegal aliens took over. In earlier centuries those folks were barbarians with limited military sophistication and could be manipulated by Rome. In the 400s they'd become more civilized and therefore much more dangerous. They ended up beating the Romans fair and square.

Then the comet hit and everything changed. Lot of effort was wasted when the Northern half of the Western Empire simply shut down (economically and socially) in or about 541 AD for the next thousand years.