The lack of resistance to the Romanian invasion convinced the Ottomans to invade the territories just ceded to Bulgaria. The main object of the invasion was the recovery of Edirne (Adrianople), which was held by Major General Vulko Velchev with a mere 4,000 troops.^[98] The majority of Bulgarian forces occupying East Thrace had been withdrawn earlier in the year to face the Serbo-Greek attack. On 12 July, Ottoman troops garrisoning Çatalca and Gelibolu reached the Enos–Midia line and on 20 July 1913 crossed the line and invaded Bulgaria.^[98] The entire Ottoman invasion force contained between 200,000 and 250,000 men under the command of Ahmed Izzet Pasha. The 1st Army was stationed at the eastern (Midia) end of the line. From east to west it was followed by the 2nd Army, 3rd Army and 4th Army, which was stationed at Gelibolu.^[98]

In the face of the advancing Ottomans, the greatly outnumbered Bulgarian forces retreated to the pre-war border. Edirne was abandoned on 19 July, but when the Ottomans did not occupy it immediately the Bulgarians re-occupied it the next day (20 July). Since it was apparent that the Ottomans were not stopping, it was abandoned a second time on 21 July and occupied by the Ottomans on 23 July.^[98]

The Ottoman armies did not stop at the old border, but crossed into Bulgarian territory. A cavalry unit advanced on Yambol and captured it on 25 July.^[98] The Ottoman invasion, more than the Romanian, incited panic among the peasantry, many of whom fled to the mountains. Among the leadership it was recognized as a complete reversal of fortune. Like the Romanians, the Ottomans suffered no combat casualties, but lost 4,000 soldiers to cholera.^[98] Some 8000 Armenians fighting for the Ottomans were wounded. The sacrifice of these Armenians was praised greatly in Turkish papers.^[99]

To help Bulgaria repulse the rapid Ottoman advance in Thrace, Russia threatened to attack the Ottoman Empire through the Caucasus, and send its Black Sea Fleet to Constantinople; this caused Britain to intervene.