[1] In August 1916, Romania chose to join the war effort on the side of the Entente and concentrated most of its forces for an invasion of Transylvania, leaving its 3rd army to guard the border with Bulgaria.

The Allies, however, still planned a large offensive in the Macedonian front for the middle of August to support Romania's entry into the war and pin down as many Bulgarian forces as possible.

The Bulgarian High Command suspected an impending offensive, and the fighting around Doiran that erupted on 9 August only confirmed these suspicions.

The need for an Allied attack against Bulgaria became even more urgent in early September 1916, as the Bulgarian Third Army under general Stefan Toshev and field marshal Mackensen achieved decisive victories against the Romanian and Russians in the battles of Tutrakan and Dobrich.

General Sarrail planned to strike at the right wing and center of the overextended First Army with his Serbian, French, Russian and Italian forces and content himself with only demonstrative attacks against the Vardar valley and the Struma, that were to be conducted by the British to pin down as many Bulgarian and German troops as possible.

The situation soon deteriorated for the Bulgarians, and on 14 September, they were forced to retreat towards Lerin, leaving behind some of their artillery guns and abandoning Gornichevo to the Serbians.

The fighting was very costly for the attackers and the defenders, as the bare, rocky ridge provided almost no cover from the Bulgarian machine gun fire or the Allied artillery.

Around that time, when it became clear that the Allies were pulling troops from the eastern flank and were concentrating them against Monastir, the commander of the Bulgarian Second Army General Todorov ordered the 7th Rila Division to take positions for an attack over the Struma river, to assist the hard pressed Bulgarians and Germans west of the Vardar.

After 4 October, the Bulgarians set up positions on the nearby heights to the east while the right flank of the 7th Rila division remained in the valley to protect the Rupel Pass.

During the following weeks, the battle developed into a series of attacks and counter-attacks in which the Allies gradually gained ground, owing to their artillery superiority.

For the duration of the battle at the Cherna Loop, some 14 Bulgarian and 4 German infantry regiments participated actively in the fighting.

The Bulgarian commander in chief, General Nikola Zhekov, protested this decision, but in the end, he couldn't stop its execution.

It came under attack almost immediately, but this time the new position held firm because the Allies were exhausted, having reached the limits of their logistical capacity.

Thus all French and Serbian attempts to break through the line were defeated, and with the onset of winter, the front stabilized along its entire length.

[6] The front was moved by only about 50km at a heavy price, and in the end, the offensive did not prevent the defeat of Romania or knock Bulgaria out of the war.

The line here remained intact until the end of the war in Macedonia when the forces occupying it had to retreat due to the breakthrough at Dobro Pole.

General Nikola Zhekov went as far as to describe the Battle of the River Cherna as "legendary" in terms of the tenacity of the Bulgarian defence – "conducted without regard of casualties.