 On 20 September, the 17th and 122nd French Colonial Divisions along with the 1st Serbian Army crossed river Crna. News of a breakthrough at Dobro Pole prompted the defenders of Doiran to abandon their positions and rush to the defense of their homeland, in order to prevent a future occupation by the Entente. On 21 September, the Allies became aware of the Bulgarian withdrawal after observing a series of fires and munition dump explosions on the Bulgarian positions, a pursuit by the British XII Corps was launched immediately. The Serbian advanced guard approached Krivolak, thus creating a wedge between the 1st Bulgarian and 11th German Armies in an effort to force the latter to retire towards Albania. The 2nd Bulgarian Army headed towards the Kosturino Pass avoiding direct engagement with the Allies.
