The Battle of Kosturino was a World War I battle, fought between 6 and 12 December 1915. The battle was fought in the initial stage of the Macedonian campaign, in the Balkans Theatre. On 6 December, a Bulgarian troops attacked the French and British-held trenches in Kosturino, at the time part of the Kingdom of Serbia (present day North Macedonia). The offensive was at first held in check, however on 8 December, Bulgaria managed to infiltrate the Memesli ravine. Bulgaria then seized Crete Simonet, thus threatening to outflank the Allies. The Entente defeat at Kosturino led to the complete withdrawal of Allied forces from Serbia, thus enabling the Central Powers to build the Berlin to Constantinople rail line. The Allies in the meantime concentrated on solidifying their defenses in Greece. Prelude The 28 June 1914, assassination of Austro-Hungarian heir presumptive Archduke Franz Ferdinand precipitated Austria-Hungary's declaration of war against Serbia. The conflict quickly attracted the involvement of all major European countries, pitting the Central Powers against the Entente coalition and starting World War I. After the entry of the Ottoman Empire into the war on the side of the Central Powers (November 1914), the decisive factor in the Balkans became the attitude of Bulgaria. Bulgaria occupied a strategically important position on the Serbian flank and its intervention on either side of the belligerents would be decisive. Bulgaria and Serbia had fought each other twice in the previous thirty years: in the Serbo-Bulgarian War of 1885 and in the Second Balkan War of 1913. Bulgaria had suffered defeat in 1913 and the Bulgarian government and people generally felt that Serbia had stolen land which rightfully belonged to Bulgaria. While the Allies could only offer Bulgaria small territorial concessions from Serbia and neutral Greece, the Central Powers' promises appeared far more enticing, as they offered to cede most of the land which Bulgaria claimed. With the Allied defeats at the Battle of Gallipoli (April 1915 to January 1916) and the Russian defeat at Gorlice-Tarnów (May to September 1915) demonstrating the Central Powers' strength, King Ferdinand signed a treaty with Germany and on 21 September 1915 Bulgaria began mobilizing for war. After the victory of the Serbian army in the Battle of Kolubara in December 1914, the Serbian front saw a lull until the early autumn of 1915. Under the command of Field Marshal August von Mackensen, the Austro-Hungarian Balkan Army, the German