There were a lot of factors, but it mainly was caused by the breakdown of leadership in the Roman Empire. After Julius Caesar was assassinated, the conspirators were unable to regain leadership and eventually all died or killed themselves during the civil war, in which Octavius or Augustus Caesar (Caesar's adopted son) and Mark Antony took back leadership. Eventually, their rule fell apart and other outside forces tried and succeeded in undermining the trust the Roman citizens had in the Roman government, and those outside forces easily defeated Rome, causing it to fall apart forever.