John I Tzimiskes was the senior Byzantine Emperor from 11 December 969 to 10 January 976. An intuitive and successful general, he strengthened the Empire and expanded its borders during his short reign. 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'. In 972, Tzimiskes turned against the Abbasid Empire and its vassals, beginning with an invasion of Upper Mesopotamia. A second campaign, in 975, was aimed at Syria, where his forces took Homs, Baalbek, Damascus, Tiberias, Nazareth, Caesarea, Sidon, Beirut, Byblos, and Tripoli, but they failed to take Jerusalem.