Because the Bulgars had been raiding Byzantine lands since 976, the Byzantine government sought to cause dissension among them by allowing the escape of their captive emperor Boris II of Bulgaria. This ploy failed so Basil used a respite from his conflict with the nobility to lead a 30,000-strong army into Bulgaria and besiege Sredets (Sofia) in 986. Taking losses and worried about the loyalty of some of his governors, Basil lifted the siege and returned for Thrace but he fell into an ambush and suffered a serious defeat at the Battle of the Gates of Trajan.

The Battle of the Gates of Trajan was a battle between Byzantine and Bulgarian forces in the year 986. It took place in the pass of the same name, modern Trayanovi Vrata, in Sofia Province, Bulgaria. It was the largest defeat of the Byzantines under Emperor Basil II. After the unsuccessful siege of Sofia he retreated to Thrace, but was surrounded by the Bulgarian army under the command of Samuil in the Sredna Gora mountains. The Byzantine army was annihilated and Basil himself barely escaped.