War against Byzantium The sudden death of George Terter II had been followed by a brief period of confusion and uncertainty, which was exploited by the Byzantine emperor
Andronikos III Palaiologos. The Byzantines overran northeastern
Thrace and captured a number of important cities including
Yambol,
Lardea,
Ktenia,
Rusokastro,
Anchialus,
Sozopol and
Agatopol. At the same time, a Byzantine-sponsored pretender,
Voysil, brother of the former Bulgarian emperor
Smilets (r. 1292-1298), ensconced himself in
Krăn, controlling the valleys between the
Balkan mountains and
Sredna Gora from
Sliven to
Kopsis. At this point the newly elected Michael Shishman marched south against Andronikos III, while another Byzantine army led by Andronikos III himself was besieging Philippopolis (
Plovdiv). Defended by a Bulgarian garrison led by
Ivan the Russian, the siege was a failure despite the Byzantines use of a 100-soldier, five-story siege tower. While the Byzantine army was engaged at Philipopolis, Michael Shishman led his troops to north-eastern Thrace and quickly retook the lost cities thus forcing the Byzantines to pull back. Despite the loss, Michael Shishman was able to expel Voysil and fully recover Bulgarian control over northern and northeastern Thrace in 1324 which had been taken by the Byzantines in the previous year during the interregnum. Again in 1324, the Bulgarian emperor invaded Byzantium advancing as far as
Traianopolis and Vira in the lower course of the
Maritsa river. Andronikos III was unable to engage the Bulgarian army because his troops were outnumbered. He offered Michael Shishman a
duel to solve the conflict. The Bulgarian emperor answered with the words cited by
John Kantakouzenos: The Byzantine emperor was said to have been infuriated with the answer and the fact that he was outsmarted. However, Michael III who was informed of the conflict between Andronikos III and Andronikos II hinted him that he could help Andronikos III against his grandfather in case of war and returned to Bulgaria promising that soon he would begin negotiations. The exact reasons for that act are unclear. Many historians suggest that the deterioration of the Bulgarian-Serbian relations was rooted in the Serbian penetration in
Macedonia. The marriage cemented the peace treaty with the Byzantine Empire but the need for an ally against the Serbs made Michael Shishman prone to make concessions. It was decided that the border should follow the Philippopolis-
Chernomen-Sozopol line. In 1327 Michael Shishman became involved in the renewed civil war in the
Byzantine Empire, taking the side of his brother-in-law Andronikos III, while his grandfather and rival
Andronikos II obtained the support of the Serbian king. Andronikos III and Michael Shishman
met at Chernomen (according to
Nicephorus Gregoras at
Dimotika) and concluded an aggressive alliance against Serbia. The Byzantine emperor promised to Bulgaria territory with several towns and large amount of money if he would become a sole emperor. Based on that alliance, Andronikos III gained control of Macedonia but his success made Michael Shishman, who aimed at a prolonged conflict within the Byzantine Empire, enter into negotiations with Andronikos II, offering military support in exchange for money and the cession of some border lands. The Bulgarian ruler sent a detachment of 3,000 cavalry, commanded by
Ivan the Russian, from Yambol to guard the Imperial Palace in Constantinople and Andronikos II but his intentions were to capture the old emperor and the city. Forewarned by his grandson, Andronikos II prudently kept the Bulgarians away from the capital and his person. When Michael Shishman understood that his plans were revealed he sent Ivan a letter with a single feather to retreat which meant that the orders had to be promptly executed. Following the victory of Andronikos III over his grandfather, Michael Shishman attempted to gain some lands by force. He invaded Thrace in June 1328 and pillaged the vicinities of
Viza but retreated before the advance of Andronikos III. Another showdown in front of
Adrianople 60 days later ended without battle and with the renewal of the peace treaty in October 1328, after which Michael Shishman returned to his country, but not before securing a large payoff. In return, the Bulgarians gave back the fortress of
Bukelon which they had taken during the initial stages of the campaign. At the beginning of the next year the Bulgarian emperor requested a personal meeting with his Byzantine counterpart to negotiate a definitive treaty and joint military operations against the growing power of Serbia. In the locality known as
Krimni between Sozopol and Anchialus the two signed "lasting peace and eternal alliance". ==Relations with Serbia==