In the following years, numerous generals of the Roman army fought each other for control of the empire and neglected their duties of defending it from invasion. There were frequent raids across the Rhine and Danube frontier by foreign tribes, including the Carpians, Goths, Vandals, and Alamanni, and attacks from Sassanids in the east. Climate changes and a sea level rise disrupted the agriculture of what is now the Low Countries, forcing tribes residing in the region to migrate into Roman lands. Further disruption arose in 251, when the Plague of Cyprian (possibly smallpox) broke out. This plague caused large-scale death, severely weakening the empire. The situation was worsened in 260 when the emperor Valerian was captured in battle by the Sassanids (he later died in captivity).