 Complete the following phrase:  The Bulgarians and the Byzantines agreed to a joint campaign against Serbia, but it took five years until the differences and tensions between Bulgaria and Byzantium were overcome. Michael Shishman gathered 15,000 troops and invaded Serbia. He engaged the Serbian king Stephen Dečanski, who commanded an approximately equal force, near the border town of Velbazhd. The two rulers, both expecting reinforcements, agreed to a one-day truce but when a Catalan detachment under the kings son Stephen Dušan arrived, the Serbs broke their word. The Bulgarians were defeated in the ensuing 