Invasion of Constantiople
The retirement of Belisarius came to an end in 559, when an army of Kutrigur Bulgars under Khan Zabergan crossed the Danube River to invade Roman territory for the first time and threatened Constantinople, Justinian recalled Belisarius to command the Roman army, in his last campaign, Belisarius defeated the Kutrigurs and drove them back across the river with the greatly outnumbered force under his command