On 21 October, Bulgarian troops attacked the French-held positions near the Strumica rail station, at the time part of the Kingdom of Serbia (present-day North Macedonia), starting the battle.

The Entente defeat at Krivolak and the follow-up battle of Kosturino led to the complete withdrawal of Allied forces from Serbia, thus enabling the Central Powers to re-open the Berlin to Constantinople rail line.

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

The Bulgarian forces occupied Kumanovo, Štip, and Skopje and prevented the withdrawal of the Serbian army to the Greek border and Salonika.

With the Bulgarian breakthrough, the Serbian position became hopeless; their main army in the north faced either encirclement and enforced surrender or retreat.

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 managed to reach the Adriatic coast and embark on Italian transport ships that carried the army to Corfu and other Greek islands before it travelled on to Thessaloniki.

From the 15–16 October, the French 57th, 122nd and 156th divisions, including two regiments of Chasseurs d'Afrique and one of Zouaves, reached the Gevgelija and Strumica rail stations.

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.

By 24 October, the 2nd Bulgarian army under Generals Georgi Todorov and Stefan Bogdanov had seized Veles and Kumanovo, cutting off the bulk of the retreating Serbs from the French.

[12] Finding the terrain impracticable for field artillery, the 57th and 122nd Divisions abandoned it at Karahojali, instead advancing towards Veles to strike the Bulgarian rear.

On 11 November, a second French attack on Fortin Bulgare and Hill 526 fared better, capturing the two positions and forcing the Bulgarian batteries on the Kosturino–Strumica road to fall back.

On the morning of 12 November, General Stefan Bogdanov ordered an offensive on Mrzen, Gradsko and the two Cicevos, believing that they were still occupied by the Allies, by the afternoon Krusevica and Grasko had been conquered by the Bulgarians.

To the west, the French reached the outskirts of Ormanli and Kosturino, halting after receiving news of Bulgarian reinforcements at Mount Belasica.

On 26 November, 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 began the Battle of Kosturino when they 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.

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.

[23] 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.

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.

[26] 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 the 7 December, and ruses, such as William Scurry's self-firing rifle, rigged to fire by water dripped into a pan attached to the trigger, were used to disguise the Allied departure.