The Battle of Kleidion (or Clidium, after the medieval name of the village of Klyuch, "(the) key"; also known as the Battle of Belasitsa) takes place on July 29, 1014 between the Bulgarian Empire and the Byzantine Empire.

It is the culmination of the nearly half-century struggle between the Bulgarian Emperor Samuel and the Byzantine Emperor Basil II in the late 10th and early 11th centuries.

The result is a decisive Byzantine victory.The battle takes place in the valley between the mountains of Belasitsa and Ograzhden near the modern Bulgarian village of Klyuch.

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

The ensuing battle is a major defeat for the Bulgarians.

Bulgarian soldiers are captured and blinded by order of Basil II, who will subsequently be known as the "Bulgar-Slayer".

Samuel survives the battle, but dies two months later from a heart attack, reportedly brought on by the sight of his blind soldiers.Although the engagement does not end the First Bulgarian Empire, the Battle of Kleidion reduces its ability to resist Byzantine advances and can be considered the pivotal encounter of the war with Byzantium.

The heirs of Samuel cannot subsequently hold off the Byzantine advance, and in 1018 the Bulgarian Empire is finally destroyed by Basil II.