The decisive encounter occurred on 29 July with an attack in the rear by a force under the Byzantine general Nikephoros Xiphias, who had infiltrated the Bulgarian positions.

Although the engagement did not end the First Bulgarian Empire, the Battle of Kleidion reduced its ability to resist Byzantine advances, and it has been considered the pivotal encounter of the war with Byzantium.

The origins of the conflict date back to the 7th century, when the Bulgars under Khan Asparukh established a state along the Danube in one of the provinces of the Eastern Roman Empire.

This war had resulted in the Bulgarian emperor Boris II being forced to renounce his imperial title in Constantinople, and eastern Bulgaria came under Byzantine rule.

Basil II's first campaign was disastrous, the Emperor barely escaping with his life when the Bulgarians annihilated the Byzantine army in the Gates of Trajan Pass in 986.

[7] Over the next fifteen years, while Basil was preoccupied with revolts against his rule and the Fatimid threat in the east, Samuel retook most of the previously conquered Bulgarian lands and carried the war into enemy territory in a series of campaigns.

Over these and the next few years, a regular pattern emerged: the Byzantines would campaign in Bulgaria, laying siege to forts and pillaging the countryside, while the numerically inferior Bulgarians, unable to offer direct opposition, launched diversionary raids in Macedonia and Greece.

A counter-attack in 1009 failed at the Battle of Kreta,[8][9] and although the Byzantines themselves did not achieve any decisive success, their methodical war of attrition deprived the Bulgarians of their strongholds and gradually weakened their forces.

[10] In the words of Byzantine historian John Skylitzes: "The Emperor Basil II continued to invade Bulgaria each year and destroy and devastate everything on his way.

In addition, the Bulgarian ruler chose Strumitsa for his defensive base — it was located on the road from Thessaloniki leading to Thrace to the east and Ohrid to the west.

[17] Basil II also prepared carefully, assembling a large army of his own and taking his most experienced commanders, including the governor of Philippopolis (modern Plovdiv), Nicephorus Xiphias, who had conquered the old Bulgarian capitals Pliska and Preslav from Samuel in 1001.

[citation needed] The Byzantine army marched from Constantinople through Komotini, Drama and Serres and reached the Rupel pass on the Struma river.

Samuel and his son Gabriel Radomir immediately headed to the east from their headquarters in the Strumitsa fortress to aid their army, but in desperate fighting near the village of Mokrievo (present-day North Macedonia) they were overwhelmed by the quickly advancing enemy.

On the return, the eloquence of the cubicularius Sergius convinced the defenders of Melnik to surrender,[37] another heavy blow for the Bulgarians as the town guarded the main road to Sofia from the south.

[45] His heirs Gabriel Radomir and Ivan Vladislav were unable to effectively resist the attacks of Basil II, and Bulgaria was completely defeated in 1018.

[46] In that year Emperor Ivan Vladislav was killed in a battle at Dyrrhachium,[47] and Bulgaria became a province of the Byzantine Empire until the successful uprising led by the Asen brothers in 1185.

The ability of the central government to control the peripheral and interior provinces of the Empire was reduced and the actions of the local and provincial governors became more decisive for the outcome of the war with Byzantium.