Many theories have been advanced in way of explanation for decline of the Roman Empire, and many dates given for its fall, from the onset of its decline in the 3rd century to the fall of Constantinople in 1453. Militarily, however, the Empire finally fell after first being overrun by various non-Roman peoples and then having its heart in Italy seized by Germanic troops in a revolt. The historicity and exact dates are uncertain, and some historians do not consider that the Empire fell at this point.