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 December 6, a Bulgarian troops attacked the French and British-held trenches in Kosturino, at the time part of the Kingdom of Serbia (present day Republic of Macedonia). The offensive was at first held in check, however on December 8, 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 Gr