During his short reign the Byzantine emperor Alexander (r. 912–913) provoked a conflict with the Bulgarian monarch Simeon I (r. 893–927). Simeon I, who had long harboured ambitions to claim an imperial title for himself, took the opportunity to wage war. With the Byzantine Empire in disarray following Alexander's death in June 913, the Bulgarians reached Constantinople unopposed and forced the regency of the infant Constantine VII (r. 913–959) to recognize Simeon I as emperor (in Bulgarian, "Tsar"). Following a palace coup in 914, the new Byzantine regency revoked the concessions to the Bulgarians and summoned the whole army, including the troops in Asia Minor, to deal with the Bulgarian threat once and all. In the decisive battle of Achelous in 917 the Byzantine forces were completely annihilated, leaving the Bulgarians in charge of the Balkans. Their annual campaigns reached the walls of Constantinople and the Isthmus of Corinth. All subsequent attempts to confront the Bulgarian army at Katasyrtai, Aquae Calidae and Pegae ended in defeat.