Boril and Theodore I campaign against the Byzantines

Tsar Boril of Bulgaria and Emperor Theodore I Laskaris formed an alliance to wage war against Constantinople. The Byzantines threatened the city by sea, while the Bulgarian army advanced overland. Despite their efforts, the campaign was probably too ambitious and difficult to carry out. The Latin emperor Henry repelled the Bulgarian army, which withdrew from Thrace. The knights then turned to attacking the Empire of Nicaea in Asia Minor.