As tensions with the Balkan League grew at however over the summer of 1912, he was switched to command the VIII Provisional Corps at Thessaloniki. After the outbreak of the First Balkan War, he led his forces against the Greek Army of Thessaly under Crown Prince Constantine. The Greek army, better prepared and outnumbering his own forces, defeated VIII Corps in the battles of Sarantaporo and Yenidje. Surrounded and blockaded in Thessaloniki and with no hope of outside succor, and learning of the approach of the 7th Bulgarian Division from the northeast, Hasan Tahsin resolved to surrender the Thessaloniki fortress and his 26,000 men to the Greeks. After a few days of negotiations, a surrender protocol was signed on , with the handover carried out the next day. The Ottoman side immediately considered him a traitor and a court martial gave a death sentence.