The book is famous not only because it is extraordinarily well written, but also because Gibbon offers an explanation for why the Roman Empire fell. This is one of the great historical questions, and, because of the relative lack of written records from the time, one of the most difficult to undertake. Gibbon was not the first to theorise about this. In fact most of his ideas are directly taken from Roman moralists of the 4th and 5th centuries who wrote about it at the time; nor would he be the last; see for example Henri Pirenne's Thesis of the early 20th century.
According to Gibbon, the Roman Empire succumbed to barbarian invasions because of a loss of civic virtue among its citizens.[3] They had become lazy and soft, outsourcing their duties to defend their Empire to barbarian mercenaries, (BLACKWATER) who then became so numerous and ingrained that they were able to take over the Empire. Romans, he believed, had become effeminate, unwilling to live a tougher, "manly" military lifestyle.