On 6 December, Bulgarian troops attacked the French and British-held trenches in Kosturino, at the time part of the Kingdom of Serbia (present-day North Macedonia).

The Entente defeat at Kosturino led to the complete withdrawal of Allied forces from Serbia, thus enabling the Central Powers to build the Berlin to Constantinople rail line.

The 28 June 1914, assassination of Austro-Hungarian heir presumptive Archduke Franz Ferdinand precipitated Austria-Hungary's declaration of war against Serbia.

The conflict quickly attracted the involvement of all major European countries, pitting the Central Powers against the Entente coalition and starting World War I.

After the entry of the Ottoman Empire into the war on the side of the Central Powers (November 1914), the decisive factor in the Balkans became the attitude of Bulgaria.

The Entente was also delayed due to protracted secret negotiations to bring Bulgaria into the Allied camp, which would have alleviated Serbia's need for Franco-British help.

The Serbs faced great difficulties: terrible weather, poor roads and the need for the army to help the tens of thousands of civilians who retreated with them.

Only c. 125,000 Serbian soldiers reached the Adriatic coast and embarked on Italian transport ships that carried the army to Corfu and other Greek islands before it travelled on to Thessaloniki.

This advance aided the retreating Serbian army, as the Bulgarians had to concentrate larger forces on their southern flank to deal with a possible invasion of their territory.

The French slowed down the Bulgarian advance, thus enabling the bulk of the Serbians to escape; however, Bulgaria retained the dominant mount Arkhangel west of Gradsko, forcing Sarrail to initiate a retreat towards Thessaloniki.

On the same day, the French forces that were previously in touch with the British at the Kajali ravine began their evacuation, dangerously exposing the left flank of the 10th Irish Division.

Half an hour later, small bands of Bulgarian troops attempted to make their way down the ridge in front of the British trench before being stopped at 60 yards (55 m) from the wire.

[13] Before the dawn of 7 December, the Bulgarians utilized the heavy fog that covered the battlefield, sneaking up to Rocky Peak and engaging their adversaries with bayonets.

At 5:45 p.m., the Bulgarian army overtook Crete Simonet seizing ten artillery pieces and celebrating their victory by blowing bugles and launching flares.

[17][18] The battle at Kosturino and the subsequent evacuation of the Allies resulted in 1,209 British casualties, including 99 killed, 386 wounded, 724 missing and ten artillery pieces.

This resulted in them opening the railway line from Berlin to Constantinople, allowing Germany to prop up its weaker partner, the Ottoman Empire.

Despite the Central Powers' victory, the Allies managed to save a part of the Serbian army, which, although battered, seriously reduced and almost unarmed, escaped destruction and, after reorganizing, resumed operations six months later.

The Allies were able to concentrate on fortifying the so-called Entrenched Camp in preparation for a large-scale Bulgarian invasion of Greece and an impending assault on Salonika.

Ottoman success began to affect public opinion in the United Kingdom, with news discrediting Ian Standish Monteith Hamilton's performance being smuggled out by journalists like Keith Murdoch and Ellis Ashmead-Bartlett.

[23] Autumn and winter brought relief from the heat but also led to gales, blizzards and flooding, resulting in men drowning and freezing to death, while thousands suffered frostbite.

Troop numbers had been slowly reduced since 7 December 1915, and ruses, such as William Scurry's self-firing rifle,[28] which had been rigged to fire by water dripped into a pan attached to the trigger, were used to disguise the Allied departure.