The First Balkan War broke out in October 1912, in which he participated as an ordinary soldier, passing from Kumanovo to Bitola. After the end of the war and liberation of Serb territories, he returned to his business, but was stopped by Austro-Hungarian threats and World War I. He immediately volunteered and joined the Jadar Detachment, commanded by veteran Vojin Popović (known as "Vojvoda Vuk"). He participated at the great victory at Kolubara, and for his bravery and able military conduct he was elevated to reserve sublieutenant of the Serbian Army. In September 1915, when the Central Powers with full force broke down Serbia, wearied by previous battles and epidemic typhus fever, Kosta Vojinović found himself in the renewed detachment of Vojvoda Vuk which was forwarded to the Čemernik mountain towards Bulgaria, to provide security for the Serbian Armies. While fighting battles there, he was wounded but did not leave the battlefield, the detachment retreated to villages around Vučitrn. There, he was taken in and promised ("besa") by Albanian Ali Šalja, who illegally transferred him to his father, who had up until the occupation worked as a president of the municipality of Kosovska Mitrovica.