Both states prospered during this interlude, but the balance of power gradually shifted in favour of the Byzantines, who made great territorial gains against the Abbasid Caliphate in the east and formed a web of alliances surrounding Bulgaria.

By 965/966, the warlike new Byzantine emperor Nikephoros II Phokas refused to renew the annual tribute that was part of the peace agreement and declared war on Bulgaria.

Preoccupied with his campaigns in the East, Nikephoros resolved to fight the war by proxy and invited the Rus' ruler Sviatoslav to invade Bulgaria.

Sviatoslav intended to continue his drive south against Byzantium itself, which regarded the establishment of a new and powerful Russo-Bulgarian state in the Balkans with great concern.

After stopping a Rus' advance through Thrace at the Battle of Arcadiopolis in 970, the Byzantine emperor John I Tzimiskes led an army north into Bulgaria in 971 and captured Preslav, the capital.

However, most of the country in the central and western Balkans remained in effect outside imperial control; this would lead to the revival of the Bulgarian state in these regions under the Cometopuli dynasty.

The early decades of the century were dominated by Tsar Simeon (r. 893–927), who expanded his empire at Byzantium's expense in a series of wars and secured for himself recognition of his imperial title.

Simeon's second son and successor, Peter I (r. 927–969), married Maria, the granddaughter of the Byzantine emperor Romanos I Lekapenos (r. 920–944), and his imperial title was recognized.

An annual tribute (which the Byzantines termed a subsidy for Maria's upkeep, to save face) was agreed to be paid to the Bulgarian ruler in exchange for peace.

Peter's reign, although lacking the military splendour of Simeon's, was still a "golden age" for Bulgaria, with a flourishing economy and a thriving urban society.

[4][5][6] Byzantium used the peace to focus its energy on wars against the Abbasid Caliphate in the East, where a series of campaigns under generals John Kourkouas and Nikephoros Phokas greatly expanded imperial territory.

[7][8] The Byzantines did not neglect the Balkans, working steadily to improve their contacts with the peoples of central and eastern Europe, subtly altering the balance of power in the peninsula.

Their Crimean outpost of Cherson maintained trade with the Pechenegs and the emerging power of the Kievan Rus'; Byzantine missionaries led the Christianization of the Magyars; and the Slavic princes of the western Balkans came to once again acknowledge the suzerainty of the Empire,[9][10] particularly after Časlav Klonimirović ended Bulgarian control over Serbia.

[14] Nikephoros, a prominent member of the Anatolian military aristocracy, also focused mostly on the East, leading his army personally in campaigns that recovered Cyprus and Cilicia.

With promises of rich rewards and, according to Leo the Deacon, a payment of 1,500 pounds of gold, the Rus' ruler was induced to attack Bulgaria from the north.

[18][20][28] The next year, Sviatoslav left with part of his army to counter a Pecheneg attack on his capital at Kiev (incited either by the Byzantines or, according to the Primary Chronicle, by the Bulgarians).

Bulgarian soldiers joined his army in considerable numbers, tempted partly by the prospects of booty, but also enticed by Sviatoslav's anti-Byzantine designs and probably mollified by a shared Slavic heritage.

[38] Thus Nikephoros' scheme had backfired: Instead of a weak Bulgaria, a new and warlike nation had been established at the Empire's northern border, and Sviatoslav showed every intention of continuing his advance south into Byzantium.

The Rus' withdrew north of the Balkan mountain range, which gave Tzimiskes time to deal with internal unrest and to assemble his forces.

[39][45] After being occupied with suppressing the revolt of Bardas Phokas throughout the year 970, Tzimiskes marshalled his forces in early 971 for a campaign against the Rus', moving his troops from Asia to Thrace and gathering supplies and siege equipment.

The Byzantine navy accompanied the expedition, tasked with carrying troops to effect a landing in the enemy's rear and to cut off their retreat across the Danube.

[50][52] Tzimiskes and Sviatoslav met and agreed to a peace treaty: The Rus' army was allowed to depart, leaving their captives and plunder behind, and their trading rights were re-affirmed in exchange for an oath to never again attack imperial territory.

This became evident during his triumphal return to Constantinople, where the emperor entered the Golden Gate behind a wagon carrying an icon of the Virgin Mary as well as the Bulgarian regalia, with Boris and his family following behind Tzimiskes.

When the procession reached the Forum of Constantine, Boris was publicly divested of his imperial insignia, and at the church of Hagia Sophia, the Bulgarian crown was dedicated to God.

^ b: John Skylitzes names Sphangel or Sphengel as Sviatoslav's second-in-command, with Ikmor as the third most important military leader, while Leo the Deacon reverses this order.