A succession of wars were fought by Septimius Severus and his successors, generally against rivals for the throne rather than for territorial aggrandizement. Septimius Severus moved east in 195, annexing Osrhoene, but was compelled to turn west to face the usurper Clodius Albinus. In 197, the eastern campaign resumed, but, after initial successes, disaffection in the ranks twice thwarted sieges of Hatra. In 203, campaigns in Africa pushed the Roman frontier southward. In 208, northern Britain was overrun by hostile forces: Severus invaded Scotland in 209, but the campaign was abandoned on his death in 211. Severus’ son Caracalla turned to the German frontier; in 215 he launched a successful invasion of Parthia, but he was assassinated in 217. A series of civil wars ensued. In 224 the Sassanids came to power in Parthia and in 229 they took the province of Mesopotamia, but they were checked in 231 by the emperor Alexander. In 234, he too was assassinated while leading his army over the Rhine. The instability of the latter part of this period was caused by Caracalla's increase of the soldiers’ pay, which led to acute financial difficulties.