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. Basil escaped with the help of his Varangian Guard and attempted to recover his losses by turning Samuel's brother Aron against him. Aron was tempted by Basil's offer of his sister Anna in marriage, but the negotiations failed when Aron discovered the bride he was sent was an imposter. By 987, Samuel had eliminated Aron. Although the titular emperor Roman of Bulgaria was captured in 991, Basil lost Moesia to the Bulgarians.