I'm assuming that you are talking about the Roman Empire. Well, the Romans had a lot to worry about in the 3rd century. There were civil wars, usurpation, all added to the barbarian invasions. This all followed the assassination of the emperor Alexander Severus. About 20-25 Roman generals assumed imperial power and fought for leadership over all or part of the empire.

These barbarians just made it worse. The Roman border guard was pulled off of their posts to fight the civil war, while barbarians freely roamed and ravaged the frontier. The Gallic Empire eventually split the empire in two, forming what we know today as the East and West Roman Empires. Provinces in the far east, such as Israel, Syria, and Egypt broke away to form the Palmyrene Empire. When Diocletian took power in 284, he formed the Tetrarchy, government ruled by a group of individuals, to try to mend the empire together again. He could not fix the problems underlying the empire such as hyperinflation, but he let the Western Roman Empire survive for another century or so. The East stood to become the Byzantine Empire whose capital, Constantinople stood for over a millennium until Constantinople was sacked by the Ottomans.

Back to the barbarians, they caused the empire to split into two different parts. Some effects they caused, but some underlying effects in the empire took a big part too. They took over the West, forming their own kingdoms. The East survived with the leadership of the emperor Constantine.

The barbarian invasions were devastating for the Romans, as they were for the endurance of their dominance of the Mediterranean for about 1,000 years. Eventually, they were able to force the empire into submission and end their reign over Europe.