Short answers, pulled from the story.
Byzantine emperor Nikephoros II Phokas hired Sviatoslav, ruler of the Kievan Rus', to attack Bulgaria in late 966 or early 967, paying him 1,500 pounds of gold according to Leo the Deacon. Nikephoros wanted to pressure Bulgaria without committing his own forces, which were engaged in campaigns against the Abbasid Caliphate in the East.
Sviatoslav crossed the Danube into Bulgaria in August 967 or 968 and the conflict ended in 971 after a three-month siege of Dorostolon. The war involved an initial Rus' conquest, a temporary withdrawal, a second invasion in summer 969, and a Byzantine military campaign that concluded with Sviatoslav's departure.
Sviatoslav's Rus' forces won the Battle of Silistra, defeating a Bulgarian army of 30,000 men and occupying most of the Dobruja region. Following the defeat, Tsar Peter I suffered an epileptic stroke, and the historian Vasil Zlatarski recorded that Sviatoslav subsequently seized 80 towns in northeastern Bulgaria.
Boris II was captured at Preslav on the 13th of April 971 and brought to Constantinople along with the Bulgarian imperial regalia. At the Forum of Constantine, he was publicly stripped of his imperial insignia, and at the church of Hagia Sophia, the Bulgarian crown was dedicated to God, symbolically ending Bulgarian statehood in Byzantine eyes.
At Arcadiopolis in early spring 970, the Byzantine general Bardas Skleros used a feigned retreat to separate the Pecheneg contingent from the main Rus' army and destroy it in an ambush. The main Rus' force then panicked and withdrew north of the Balkan mountain range, giving Tzimiskes time to suppress internal revolts and assemble a full invasion army.
After the three-month siege of Dorostolon, Sviatoslav was forced to capitulate. Byzantine chroniclers reported that only 22,000 of an original Rus' force of 60,000 survived. Sviatoslav's army was permitted to withdraw, but had to surrender all captives and plunder and swear never to attack Byzantine territory again.