Political instability in Rome grew. Emperor Caligula (37-41) was murdered by the Praetorian Guard to replace him with Claudius (41-53), while his successor Nero (54-68) burned Rome down. The average reign from his death to Philip the Arab (244-249) was six years. Nevertheless, external expansion continued, with Trajan (98-117) invading Dacia, Parthia and Arabia. Its only formidable enemy was the Parthian Empire. Migrating peoples started exerting pressure on the borders of the empire. The drying climate of Central Asia forced the Huns to move, and in 370 they crossed Don and soon after the Danube, forcing the Goths on the move, which in turn caused other Germanic tribes to overrun Roman borders. In 293, Diocletian (284-305) appointed three rulers for different parts of the empire. It was formally divided in 395 by Theodosius I (379-395) into the Western Roman and Byzantine Empires. In 406 the northern border of the former was overrun by the Alemanni, Vandals and Suebi invaded. In 408 the Visigoths invaded Italy and then sacked Rome in 410. The final collapse of the Western Empire came in 476 with the deposal of Romulus Augustulus (475-476).