What led to the decline of the Roman Empire?
Augustus at the turn of the milennium foresaw the problem, and set about stabilising the borders on the defensible Rhine-Danube line, and in the Middle East and North Africa he worked on diplomacy.
Instability in the empire started with internal struggles for leadership. Then the successive migration westwards of Goths, Vandals, Huns, etc created problems of defence, then absorption, which delayed but did not overcome the eventual collapse of the western part of the empire, which ceased to have an emperor after 476 CE. The Empire lived on in the east centered on Constantinople, shrinking until captured by the Turks in 1453 CE.
While the internal turmoil between contestants for the emperorship was a destabilising factor, the basic fact is that the borders were simply too long and open to be defended against the hordes from the east which kept pouring in.
