On the other hand, an important dichotomy existed among the main strengths of the Entente. Britain believed that without Greece on the side of the Entente, Serbia would be lost. While France believed that a significant military presence in the southern part of the Balkans was necessary, especially after the failure of the Dardanelles, for Romania and Greece not to join the central powers. At the same time, in Thessaloniki, the Allies began to build a military base and the British and French armies of General Maurice Sarrail were already present in the region. In December 1915, the French staff sent General Noël de Castelnau on a mission to the Balkans to examine the situation on the ground and the troops of the Allies, but also the possibility of developing the Macedonian Front. Six months later, General Sarrail was appointed commander-in-chief of the Allied army at the Macedonian Front, including the army stationed in Macedonia. This Allied army controlled about 30,000 square kilometres of territory. The population of this part of Macedonia was not hostile to the Allied army, but in view of the destruction caused by previous military conflicts, they distanced themselves from them. On the other hand, the Allies had doubts about the local population, very often considered to be dedicated to the Ottoman Empire and Bulgaria. Thessaloniki was at the centre of this area, a large cosmopolitan city with its history and a cross between cultures and ethnic groups (Jews, Turks, Greeks, Macedonians, Bulgarians, Vlachs, Roma and others), which had been included in the Greek state since 1912. After conducting analyses and drawing up additional plans, the Allied army deployed north of Thessaloniki, occupying a length of about 120 km. During the first half of 1916, the line of defence expanded increasingly, with various buildings and infrastructure managed by thousands of Allied soldiers and local civilians. Trenches, wells and canals were dug, wetlands drained, roads constructed and repaired. At the same time, thousands of French soldiers were transported by ships from Marseille and Toulon (South France) or even Otranto (Southern Italy) to the port of Thessaloniki. The Zeytinlik camp was the center of this military force, located 5 km northeast of Thessaloniki, near the Vardar River.

Close to France and Great Britain, Romania agreed to join the Allies on condition of territorial expansion and greater Allied military involvement in the Balkans. The agreement provided for the Eastern Army to begin operations against the Bulgarian army in Macedonia on 20 August, which would then direct its forces towards Romania. Romania, for its part, was to declare war on Austria-Hungary and attack Hungary by 28 August at the latest. However, on 17 August and to the surprise of the Allies, Bulgarian forces attacked along the Struma River valley towards Voden. Bulgarian troops even managed to enter Florina, but their advance soon stopped. This event forced General Sarrail to change his military plans. The situation on the ground was not in favour of the Allied army, which nevertheless seemed to have superiority in terms of numbers of troops and military equipment. Many military troops were not prepared for military manoeuvres. Various epidemics seriously disrupted the Allies' preparedness for combat. In just two months (July and August 1916) approximately 11,500 French soldiers were evacuated because of various illnesses. In addition, from a logistical point of view, General Sarrail had a much more complex military structure and organization at his disposal. He led an army composed of 5 different armies (Serbs, British, French, Italian and Russian). On the other hand, to the north was a smaller but more compact military structure, composed mainly of Bulgarian troops and a small part of German troops. However, the Allies decided to stop the advance of the Bulgarian troops and launched a counter-attack in September to restore the lost positions. From his command post, French General Cordonier launched the attack on 12 September. The French, Serbian and Russian armies participated. In a few days, the Allies managed to conquer Florina and Kaimkchalan (2524 m.). The following month, the Allied forces succeeded in crossing the Black River and occupying the important Dobro Pole Massif. The conquered territories allowed the Allies to gain better control of part of the terrain and front, which facilitated the conquest of Bitola. Meanwhile, and until the end of the year, Romania was defeated, and most of its territory was occupied by Allied forces, which benefited from the large grain and oil production.

. In order to avoid these attacks, French troops tried on several occasions to suppress the Bulgarian and German armies in the north. This would have allowed the Allies to increase the perimeter, i. e. the distance between the city and the German and Bulgarian lines and their artillery positions. Several battles were fought around Bitola in this objective, for places historically known as the Red Rocks, Kota 1248, high places that dominate the city and its surroundings. The first of these offensives took place in March as part of an offensive along the front line. The offensive began on 11 March. The battles took place to the west and north of the town of Bitola. For nine days, attempts to retake Kota 1248 and the Red Rocks failed. The initial success of the Red Rocks conquest was short-lived, and Kota 1248 remained detached until the end of the war. The losses were significant, but the Front Line remained unchanged. However, the Allies managed to settle in and around Bitola.They also settled along the Front Line west of Bitola, specifically around Korca. In the following months of 1917, attacks were carried out to the west and east of Bitola, around Lake Ohrid and in Marivo, in the Cerna River loop. Today, remains of the trenches used by the two warring parties are still visible. Some of them are easily accessible and well organized for sightseeing tours, such as those of Pelister. The trenches dug deep into the rocks are visible near Kota 1050, northeast of the Bitola mining complex. Remnants of bullets, grenade launchers and other military equipment testify to the fierce battles waged on this part of the Front. Bitola has the highest loss of life and the most severe destruction. From the summer of 1916 until the end of the war, the city was bombed every day. At first by French artillery, then, from 19 November, by German and Bulgarian artillery. During these attacks, toxic military gas was also used. A large part of the town of Bitola was destroyed or burned down because of flammable grenades; 4224 people lost their lives, mostly women and children.

On 31 August, after a period of secret planning, the General Command of the Allied Forces of the Macedonian Front presented the operational plan to its officers. Preparations began the next day. Heavy artillery was placed in the highest parts of Kaymakchalan and hundreds of tons of military equipment were transported daily to the starting positions. Polish hospitals were transferred near the front, new roads were built, military maps were completed. All these activities were carried out at night to surprise the opponent. Nevertheless, thanks to their intelligence network and air investigations, the German and Bulgarian forces knew that a major offensive was being prepared. In September 1918, the 75 French and Serb battalions (with 580 cannons) were placed against the 26 German and Bulgarian battalions (with 140 cannons). On 14 September, artillery preparation was carried out in the field, after which aircraft were dispatched to examine and correct the artillery. The next day, at 5 a. m., the Allies began the offensive in the Meglen area, while secondary offensives were launched along the Front Line. The second army of Duke Stepanovic attacked with 5 divisions at the summit of Kozjak. Two French divisions (the 17th colonial and the 122nd), supported by the Shumadia division, faced the radius of Sokol, Dobro Pole and Veternik. In the evening, the Allies succeeded in taking most of these places. On the fourth day of the offensive, the strategic point Kozjak was conquered, after which the three divisions of General Bojovic and the 17th Colonial Division continued towards Gradsko. The German-Bulgarian line of defense began to collapse. General d' Espèrey then ordered to intensify the offensives on both wings of the front line of battle. To the east of the Front, British troops successfully carried out offensives in the Doiran area, while in the west French troops conquered the Dobro Pole massif. On September 21, the Front's breakthrough reached a depth of 50 km. The French and Serbian troops managed to maneuver to prevent the German and Bulgarian troops from retreating and regrouping. On September 24, Gradsko, one of the strategic locations, is conquered. The Allies succeeded in confiscating several thousand tons of military equipment and supplies, enabling them to continue their action without waiting for supplies from the south. The rest of the German and Bulgarian armies, disorganized for lack of time, then withdrew to Skopje.

Cavalry operations began on 20 September. They were composed of three regiments of African cavalry, commanded by Colonel Juino-Gambetta. Their reinforced northward march began in the vicinity of Florina, where they were initially. They conquered Prilep in just two days. The cavalry under Gambetta's command did not wait for the Serbian army to occupy Veles (which happened on September 27) and bypassed the city by mountain Goleshnica not to compromise their plan. This maneuver was to open the way to the Skopje Valley. This initiative took place during the nights from 26 to 27 December, and on 28 December they entered the village of Drachevo. The Skopje offensive began the next day, on the three sides - south, east and west - on the following day. By the end of the day, the African cavalry conquered the city. The German and Bulgarian armies tried to surrender the city, even launching artillery bombardments on the city. However, the arrival of the French troops of Veles gave up the idea of the German armies. This military operation was of enormous importance: the cavalry succeeded in taking possession of a large quantity of military equipment, but more importantly, the withdrawal of the German army was stopped. Without a way out, the German army capitulated. A total of 70,000 German soldiers and all military equipment were captured. Previously, encouraged by Dobro Pole's breakthrough, the British and Greek armies had undertaken enhanced operations against the Bulgarian army east of Doiran and Lake Doiran. Being in a desperate situation, the Bulgarian armies were forced to withdraw from the front and, on 29 September, representatives of the Bulgarian government and the army signed an armistice in Thessaloniki. After the conquest of Skopje and the defeat of Bulgaria, Serbian and French troops continued to advance in northern Serbia. On 12 October, the allies entered Niš and on 1 November, the Serbian armies entered Belgrade.