During the night of 16–17 June, the Bulgarians, without an 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, the Bulgarians 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–Lachanas line.

At Kilkis, the Bulgarians had constructed strong defenses including captured Ottoman guns which dominated the plain below. The Greek divisions attacked across the plain in rushes under Bulgarian artillery fire. On 19 June, the Greeks overran the Bulgarian forward lines everywhere but suffered heavy losses as the Bulgarian artillery fired incessantly with great accuracy guided by their observation on the hills of Kilkis. Acting under the previous order of the Greek HQ which requested Kilkis be captured by the night of 20 June, the 2nd division went forward alone. During the night of 20 June, following an artillery fire exchange, two regiments of the 2nd division crossed the Gallikos River and successively attacked the 1st, 2nd and 3rd defensive lines of the Bulgarians entering the town of Kilkis by the morning of 21 June. In the morning the rest of the Greek divisions joined the attack and the Bulgarians retreated to the north. The Greeks pursued the retreating Bulgarians but lost contact with their enemy due to exhaustion. 

The defeat of the Bulgarian 2nd Army by the Greeks was the greatest military disaster suffered by the Bulgarians in the 2nd Balkan war. On the Bulgarian right, the Evzones captured Gevgelija and the heights of Matsikovo. As a consequence, the Bulgarian line of retreat through Doiran was threatened and Ivanov's army began a desperate retreat which at times threatened to become a rout. Reinforcements came too late and joined the retreat toward Strumica and the Bulgarian border. The Greeks captured Dojran on 5 July but were unable to cut off the Bulgarian retreat through Struma Pass. On 11 July, the Greeks came in contact with the Serbs and then pushed on up the Struma River until they reached Kresna Gorge on 24 July.