It took the Roman Empire 400 years to fall into chaos. They allowed the Visigoths, Franks, Vandals, Celts and many other tribes to penetrate their villages, their army’s ranks, and Rome itself. A slow self-destruction ensued until Rome burned as Nero fiddled. The Roman Empire expired in 476 A.D.

What were the reasons?

Immigration and barbarian invasion of Italy. “After centuries of fighting the Romans, the once ill equipped and poorly organized tribes slowly became a much more formidable foe. The tactics of the Barbarians developed over time and soon what was once a trivial nuisance became a serious threat.”
Corruption: “The Roman Empire was never totally free of corruption, but it reached its zenith in the 3rd century and onward. Government officials and middlemen became much more interested in their own well-being rather than that of Rome.”
Famine: “This also affected the Barbarian tribes in the North causing them to push southward for a more stable and livable climate. As one could imagine this only exacerbated the countless issues the empire was facing at this time. Unable to care for their own citizens along with the new arrivals, many Romans starved or turned to banditry to find food. Lawlessness was widespread in the final years of the empire.”
Disloyal Military: “By the 3rd century, the legions, made up of highly dedicated and motivated Roman citizens were a distant memory. As the military expanded to keep up with constant wars and raids from its enemies, it struggled to entice Roman citizens to join the legions to protect their homelands.”
Civil Wars: “Since the time of Augustus, the Roman Empire could never figure out a functional transition of power from one emperor to the next. This often led to civil wars.”
Economic Destitution: “Rome’s constant wars with its neighbors did not come cheap. The Roman Empire struggled to pay for its military, needing to constantly raise taxes that many could not pay. This killed any semblance of a “middle class” and it also forced rich landowners to come up with ever-ingenuous ways to hide their wealth. An over-reliance on slave labor also hamstrung the empire in its later years.”
Weak and Corrupt Rulers: “The emperors that emerged victorious from disastrous power struggles were not always fit to run the most powerful empire in the world.”
If you look around America today, we suffer 50,000,000 (million) foreign-born immigrants with incompatible world views, political religions like Islam, incompatible cultures and hundreds of different languages. They lack any affinity or commitment to America. Millions of them subsist on welfare.