Rome - Kingdom, Republic and Empire together - lasted about 1200 years. Rome ruled all around the Mediterranean and large sections of Europe and Southwest Asia. Yet in the end, the problems grew too great and the Western Roman Empire fell.
Why did the empire fall apart?
There is no one single reason, but was instead a series of problems that grew too large to solve.
One problem was that the empire grew too large to govern. The Romans tried to solve this by dividing the empire into the Eastern and Western Roman Empire.
Another problem was that the Roman men no longer wanted to serve in the army. The pay was not great. Sometimes the legion was not paid at all. Most men did not want to be away from their families and farms for long periods of time. The Romans tried to solve this by hiring barbarians to fight for them.
There were other problems. Civil wars broke out in Rome between different factions. The rich grew disinterested in helping Rome and only wanted more for themselves and their families. The population of Rome shrank. Trade decreased while taxes and prices increased. The poor were overtaxed. The use of slaves put people out of work. The roads fell into disrepair.
The Romans were aware of these problems and sometimes, under a good emperor, they would work on them for a while. In time, the good emperor would be gone and a bad one would take his place. Soon the problems in the Western Roman Empire grew so great that the Western (European) Roman empire, which included the city of Rome, fell to barbarian tribes in Europe.
The Eastern Roman empire continued another 1000 years.