Byzantine Emperor Basil II was determined to capture the remaining Bulgarian lands ruled by Samuel of Bulgaria. On 29 July, after years of fighting and territory changing hands repeatedly, Basil began a march into Bulgaria and the mountainous region of the Strumitsa valley through the pass of Kleidion. Samuel knew that Basil’s army would have to travel through this area so reinforced it heavily with troops and a blockade near the village of Klyuch, west of Petrica. At first Basil’s troops attempted to break through the wooden wall blocking the road but were unsuccessful. He then ordered some of his troops to pass the wall by ascending the steep mountain slope to the south. They successfully outflanked the Bulgarian defenders, which allowed the main body of Byzantine forces to break through. The Bulgarians were then routed and driven west whilst Samuel unsuccessfully attempted to bring reinforcements down the pass.