Monastir offensive

On 12 September, the Allies opened their offensive with a powerful two-day artillery barrage and an attack by the Serbian Third Army and the French Army of the Orient against the Bulgarian 8th Tundzha Infantry Division and colonel Tasev's reinforced brigade. 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. On 12 September, the Serbians also began their first attack on the 2,300-meter-high (7,500 ft) Kaimakchalan ridge. The British also became active on the Struma front and tried to expand their footholds on its right bank.

The Bulgarian First Army's western flank now managed to hold the Allies on the Lerin – Kajmakcalan line. The Allies, however, continued their attacks, and on 23 September, after heavy fighting, the French entered Lerin. The Bulgarians were still holding on Kajmakcalan, where the 1st Infantry Brigade of the 3rd Balkan Infantry Division was attacked by a superior number of Serbian troops supported by heavy French artillery. 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.

The Bulgarian setbacks attracted greater attention from both the Bulgarian and German high commands, and soon, several vital changes in the command structure were made. On 27 September, General Kliment Boyadzhiev was replaced as commander of the First Army by General Dimitar Geshov. The army itself exchanged headquarters with the Eleventh Army of General Arnold von Winkler. This was followed by the arrival on the front of General Otto von Below and the establishment of Army Group Below on 16 October, which included both the Eleventh and First Armies.

On 30 September, after 18 days of heavy fighting, the Serbian Drina Division finally captured Kajmakcalan from the exhausted 1st infantry brigade of the 3rd Balkan Infantry Division, achieving a breakthrough in the Bulgarian defensive line. The German and Bulgarian commands regarded the loss of the position and seven artillery guns as irreversible due to the lack of a capable artillery reserve. General Winkler ordered the 8th Tundzha Divisions, the 1st and 3rd brigades of the 6th Bdin Division and the 2nd Brigade of the 9 Pleven Division to withdraw to a new defensive position. The 1st Brigade of the 3rd Balkan Division was reorganized, and its depleted nine battalions were scaled down to five, with four mixed companies with seven mountain guns and a pioneer company. The new Kenali defensive line was occupied from Lake Prespa to Kenali by the three independent infantry brigades(9/2 IB, 2/6 IB and 1/6 IB), from Kenali to the heights east of the Cherna river by the 8th division and from there to the Mala Rupa peak by the 1/3 Infantry Brigade. Further to the east were the remaining forces of the Eleventh Army – the rest of the 3rd Balkan Infantry Division, whose positions remained unchanged since they were occupied on 25 July 1916.

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. The Bulgarian high command, however, refused to permit the attack. This hesitation allowed the British to consolidate their positions on the left bank of the Struma around the village of Karacaköy on 30 September. On 3 October, the 10th (Irish) Division attacked the Bulgarian positions in the village of Yenikoy that were defended by the 13th Rila Regiment of the 7th Division. The battle lasted for the entire day, and the Bulgarians, reinforced by the 14th Macedonian Regiment and 17th Artillery Regiment, retook the village twice after a fierce bayonet struggle. During the night after a third and final attack, the Irish division occupied it. Casualties on both sides were heavy due to the accurate artillery and machine gun fire. After the battle, the Bulgarian 13th Regiment was reorganized to a three-battalion strength instead of the usual four battalions. 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. From this point on, no large operations were conducted on the Struma front until the end of the offensive.

Bulgarian infantry attack in the Monastir area.

A prime problem for the Bulgarians was that their army and resources were stretched to the limits from Dobruja to Macedonia and Albania. The Bulgarian high command turned to its German allies. The Germans themselves had few reinforcements to offer as the Brusilov offensive had taken its toll, and the Battle of the Somme was still raging. They turned to the Ottoman Empire and convinced Enver Pasha to send the 11,979 men of the 50th Division to Macedonia. In October, these forces took up position on the Struma and, a month later, were joined by the 12,609 men of the 46th Ottoman Division. The two divisions formed the XX Corps and remained in the region until May 1917, when they were recalled to Mesopotamia.[5] This freed some Bulgarian forces that could now be directed to reinforce the Eleventh Army. In addition, the Ottoman Rumeli Detachment (177th Regiment) of 3,598 men was also attached to General Winkler's forces.

On 30 September, General Joffre informed General Sarrail of the impending great offensive of the Romanian and Russian forces under General Averescu against the Bulgarian Third Army in Dobrudja and their expected crossing of the Danube between Ruse and Tutrakan. The Allied Army of the East commander planned to now use this by coordinating it with a renewed push against the Eleventh Army's Kenali line and eventually knocking Bulgaria out of the war. On 4 October, the Allies attacked the French and Russians in the direction of Monastir – Kenali, the Serbian First and Third Army along the Kenali – Cherna Loop line, the Serbian Second Army against the Third Balkan Division – in the direction of Dobro Pole. The Allies had 103 battalions and 80 batteries against the 65 battalions and 57 batteries of the Central Powers in the area.

General Dieterichs' Russian brigade on the march in Macedonia

The Battle of the River Cherna opened with the Serbians trying to gain a foothold on its northern bank. Their progress was slow, and further west, the initial French and Russian attacks were repulsed. 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. The Bulgarian and German commands also tried to stabilize the situation by reinforcing the Eleventh Army with troops transferred from the First and even the Second Army. For the duration of the battle at the Cherna Loop, some 14 Bulgarian and 4 German infantry regiments participated actively in the fighting. The French and Russians achieved a breakthrough around Kenali by the end of October but were soon halted by the Bulgarians and Germans. The Italian division was also brought to the front and supported the attacks around Monastir.

French troops in Monastir, 22 November.

By this time, however, General Below had decided to abandon Monastir, and on 18 November, while the heavy fighting was still going on, General der Infanterie Winckler ordered the Eleventh Army to retreat to new positions to the north of Monastir. The Bulgarian commander in chief, General Nikola Zhekov, protested this decision, but in the end, he couldn't stop its execution. On 19 November, French and Russian soldiers entered the town. The Bulgarians established a new position on the Chervena Stena – height 1248 – height 1050 – Makovo – Gradešnica defensive line. 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. On 11 December, General Joffre called off the offensive.