There are several factors in the eventual fall of Rome, both militarily and political. For one, larger and larger portions of the legions began to be made up of hired mercenaries from the Germanic peoples . This undermined morale, lowered combat effectiveness, and allowed generals to create personal armies loyal to them and their cash rather than to the state. This lead to the unfortunate situation where Kings would often be placed on the throne because of their popularity with the army. Additionally, over the centuries many parts of the former Empire had been ceded to local warlords, reducing the traditional buffer zones and allowing enemy armies to get much closer to Rome with less warning as well as reducing the Roman tax base. In summary, the fall was not due to strategic or tactical errors in the field of battle but the categorical weakening of the legion itself and, more importantly, the state behind it that led to the fall of the Empire.

*Footnote, this really only applies to the Western Empire and all references to "the Empire" are as such.