In our world,the Bulgarian kingdom under Michael III soundly defeated the Serbs under Stefan Dusan and killed Stefan Decanski.This happened after the Serbian army was weakened due to Decanski deciding to participate in the Byzantine civil war on the emperor side,and,while defeating the emperor grandson who fled to Tarnovo,was also weakened by the long sieges and the battles of the civil war.But what would have happened if the Serbian forces only helped the emperor by taking minor forts in Macedonia,allowing his grandson to win the civil war and to defeat the Bulgarian army at Velbazhd in 1330? How would that affect history as we know it,especially in the period between 1330 and 1355?