After his coronation in December 969, Tzimiskes dispatched his brother-in-law Bardas Skleros to subdue a rebellion by Bardas Phokas, a cousin of Tzimiskes who aspired to succeed their uncle as emperor. To solidify his position, Tzimiskes married Theodora, a daughter of Emperor Constantine VII. He proceeded to justify his usurpation by repelling the foreign invaders of the Empire. The tributary of Aleppo was soon assured under the Treaty of Safar. In a series of campaigns against the Kievan Rus' encroachment on the Lower Danube in 970–971, he drove the enemy out of Thrace in the Battle of Arcadiopolis, crossed Mt. Haemus, and besieged the fortress of Dorostolon (Silistra) on the Danube for sixty-five days, where after several hard-fought battles he defeated Great Prince Svyatoslav I of Rus'. Tzimiskes and Svyatoslav ended up negotiating a truce, in which weaponry, armor and provisions were exchanged for the famished Rus' departure. On his return to Constantinople, Tzimiskes celebrated a triumph, built the Church of Christ of the Chalke as thanksgiving, divested the captive Bulgarian Emperor Boris II of the Imperial symbols, and proclaimed Bulgaria annexed. He further secured his northern frontier by transplanting to Thrace some colonies of the Paulicians, whom he suspected of sympathising with their Muslim neighbours in the east.