What caused Rome to decline and eventually fall?

This is easily the most argued question about the fall of Rome. The Roman Empire lasted over a thousand years and represented a sophisticated and adaptive civilization. Some historians maintain that it was the split into an eastern and western empire governed by separate emperors caused Rome to fall.

Most classicists believe that a combination of factors including Christianity, decadence, the metal lead in the water supply, monetary trouble, and military problems caused the Fall of Rome.3  Imperial incompetence and chance could be added to the list. And still, others question the assumption behind the question and maintain that the Roman empire didn't fall so much as adapt to changing circumstances.

's the question?