Short answers, pulled from the story.
Sviatoslav's invasion of Bulgaria began in August 967 or 968 when the Rus' crossed the Danube into Bulgarian territory. The conflict started after Byzantine emperor Nikephoros II Phokas refused to renew an annual tribute payment and invited Sviatoslav to attack from the north.
Byzantine emperor Nikephoros II Phokas initiated the war by inviting Sviatoslav to invade Bulgaria before his death on the 11th of December 969. He was succeeded by John I Tzimiskes who led the final campaign that ended the conflict with a complete Byzantine victory.
Boris II was captured when the city of Preslav fell on the 13th of April 971 during the siege of Dorostolon. After the fall of Dorostolon, John I Tzimiskes publicly divested Boris of his imperial insignia and reduced the Bulgarian patriarchate to an archbishopric subject to Constantinople.
According to historian Vasil Zlatarski, Sviatoslav seized 80 towns in northeastern Bulgaria which were looted and destroyed but not permanently occupied at first. The Rus' army defeated a Bulgarian force of 30,000 men in the Battle of Silistra and occupied most of the Dobruja region.
The subsequent siege of Dorostolon lasted for three months during which the Byzantines blockaded the city by land and sea. The main Rus' force under Sviatoslav withdrew towards Dorostolon on the Danube before the final defeat that concluded the conflict between Kievan Rus' and the First Bulgarian Empire.