During the night of 16–17 June 1913, the Bulgarians, without official declaration of war, attacked their former Greek and the Serbian allies, and managed to evict the Serbs from Gevgelija, cutting off communication between them and the Greeks. However, they failed to drive the Serbs away from the Vardar/Axios river line. After repulsing the initial Bulgarian attack of 17 June, the Greek army, under King Constantine, advanced with 8 divisions and a cavalry brigade, while the Bulgarians under General Ivanov retreated to the naturally strong defensive position of the Kilkis-Lahana line.