Byzantine–Bulgarian wars
Constantine VI's failed retaliations

In 791, the Byzantine emperor Constantine VI embarked on an expedition against Bulgaria, in retaliation for Bulgarian incursions in the Struma valley since 789. Kardam pre-empted the Byzantine invasion and met the enemy near Adrianople in Thrace. The Byzantine army was defeated and turned to flight.

In 792, Constantine VI led another army against the Bulgars and encamped at Marcellae, near Karnobat, which he proceeded to fortify. Kardam arrived with his army on July 20 and occupied the neighboring heights. After some time passed with the two forces sizing up each other, Constantine VI ordered the attack, but in the resulting Battle of Marcellae the Byzantine forces lost formation and once again were defeated and turned to flight. Kardam captured the imperial tent and the emperor's servants. After his return to Constantinople, Constantine VI signed a peace treaty and undertook to pay an annual tribute to Bulgaria.

By 796, the imperial government was recalcitrant and Kardam found it necessary to demand the tribute while threatening to devastate Thrace if it were not paid. According to the chronicler Theophanes the Confessor, Constantine VI mocked the demand by having dung sent instead of gold as "fitting tribute" and promising to lead a new army against the elderly Kardam at Marcellae. Once again the emperor's army headed north, and once again it encountered Kardam in the vicinity of Adrianople. The armies faced each other for 17 days without entering into battle, while the two monarchs probably engaged in negotiations. In the end, conflict was averted and the peace resumed on the same terms as in 792.
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