<P> In the years following the emperor's death, generals of the Roman army fought each other for control of the empire and neglected their duties of defending the empire from invasion . Provincials became victims of frequent raids along the length of the Rhine and Danube rivers by such foreign tribes as the Carpians, Goths, Vandals, and Alamanni, and attacks from Sassanids in the east . Climate changes and a rise in sea levels ruined the agriculture of what is now the Low Countries forcing tribes to migrate . Additionally, in 251, the Plague of Cyprian (possibly smallpox) broke out, causing large - scale death, and possibly weakened the ability of the empire to defend itself . </P> <P> After the loss of Valerian in 260, the Roman Empire was beset by usurpers, who broke it up into three competing states . The Roman provinces of Gaul, Britain, and Hispania broke off to form the Gallic Empire . After the death of Odaenathus in 267, the eastern provinces of Syria, Palestine, and Aegyptus became independent as the Palmyrene Empire, leaving the remaining Italian - centered Roman Empire - proper in the middle . </P> <P> An invasion by a vast host of Goths was defeated at the Battle of Naissus in 268 or 269 . This victory was significant as the turning point of the crisis, when a series of tough, energetic soldier - emperors took power . Victories by the emperor Claudius II Gothicus over the next two years drove back the Alamanni and recovered Hispania from the Gallic Empire . When Claudius died in 270 of the plague, Aurelian, who had commanded the cavalry at Naissus, succeeded him as the emperor and continued the restoration of the Empire . </P> <P> Aurelian reigned (270--275) through the worst of the crisis, defeating the Vandals, Visigoths, Palmyrenes, Persians, and then the remainder of the Gallic Empire . By late 274, the Roman Empire was reunited into a single entity, and the frontier troops were back in place . More than a century would pass before Rome again lost military ascendancy over its external enemies, however, dozens of formerly thriving cities, especially in the Western Empire, had been ruined, their populations dispersed and, with the breakdown of the economic system, could not be rebuilt . Major cities and towns, including Rome itself, had not needed fortifications for many centuries, but then surrounded themselves with thick walls . </P>

Who invaded roman territory in the third century