Conversation so far:
Can you tell me about Vlachs in the 13th century?
In 1213 an army of Romans (Vlachs), Transylvanian Saxons, and Pechenegs, led by Ioachim of Sibiu, attacked the Bulgars and Cumans from Vidin.

Question: Can you tell me what led to this?
Self-contained rewrite: Can you tell me what led to the Vlachs, Saxons, and Pechenegs attacking the Bulgars and Cumans in 1213?