Macedonian front

The Macedonian front, also known as the Salonica front (after Thessaloniki), was a military theatre of World War I formed as a result of an attempt by the Allied Powers to aid Serbia, in the autumn of 1915, against the combined attack of Germany, Austria-Hungary and Bulgaria. The expedition came too late and with insufficient force to prevent the fall of Serbia and was complicated by the internal political crisis in Greece (the National Schism). Eventually, a stable front was established, running from the Albanian Adriatic coast to the Struma River, pitting a multinational Allied force against the Bulgarian army, which was at various times bolstered with smaller units from the other Central Powers. The Macedonian front remained stable, despite local actions, until the Allied offensive in September 1918 resulted in Bulgaria capitulating and the liberation of Serbia.

Following the assassination of Archduke Franz Ferdinand by a Bosnian Serb, Austria-Hungary had attacked Serbia in August 1914 but had failed to overcome Serbian resistance. 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. Bulgaria occupied a strategically important position on the Serbian flank, and its intervention on either side of the belligerents would be decisive. Bulgaria and Serbia had fought each other twice in the previous thirty years: in the Serbo-Bulgarian War of 1885 and the Second Balkan War of 1913. Bulgaria had suffered defeat in 1913, and the Bulgarian government and people generally felt that Serbia had seized land which rightfully belonged to Bulgaria. While the Allies could only offer Bulgaria small territorial concessions from Serbia and neutral Greece, the Central Powers' promises appeared far more enticing, offering to cede most of the land Bulgaria claimed. With the Allied defeats at the Battle of Gallipoli (April 1915 to January 1916) and the Russian defeat at Gorlice-Tarnów (May to September 1915) demonstrating the Central Powers' strength, King Ferdinand signed a treaty with Germany and on 21 September 1915 Bulgaria began mobilizing for war.

After the victory of the Royal Serbian Army in the Battle of Kolubara in December 1914, the Serbian front saw a lull until the early autumn of 1915. Under the command of Field Marshal August von Mackensen, the Austro-Hungarian Balkan Army, the German 11th Army and river flotillas on the Danube and the Sava began an offensive on 6 October 1915, the largest offensive against Serbia. By September 1915, despite the extreme sacrifice of the Serbian army, the Austro-Hungarian Balkan Army, having crossed the rivers Sava and Drina, and the German 11th Army after crossing the Danube, occupied Belgrade, Smederevo, Požarevac and Golubac, creating a vast bridgehead south of the Sava and Danube rivers, and forcing Serbian forces to withdraw to southern Serbia.

On 15 October 1915, two Bulgarian armies attacked, over-running Serbian units and penetrating the valley of the South Morava river near Vranje up to 22 October 1915. The Bulgarian forces occupied Kumanovo, Štip, and Skopje and prevented the withdrawal of the Serbian army to the Greek border and Thessaloniki (Salonika).

The Allies (Britain and France) had repeatedly promised to send military forces to Serbia, but nothing had materialized for a year. However, with Bulgaria's mobilization to its south, the situation for Serbia became desperate. The developments finally forced the French and the British to decide upon sending a small expedition force of two divisions from Gallipoli (156th Infantry Division (France) and 10th (Irish) Division respectively). Though the first troops landed in the port of Salonika on 5 October to combine into an Army of the Orient under the French commander Maurice Sarrail, they arrived in the Greek port of Thessaloniki too late to contribute to the operations to help Serbia. The main reason for the delay was the lack of available Allied forces due to the critical situation in the Western Front. The Allies used Greek neutrality as an excuse^{[citation needed]}, although they could have used the Albanian coast to rapidly deploy reinforcements and equipment during the first 14 months of the war. (As the Serbian Marshal Radomir Putnik had suggested, the Royal Montenegrin Army gave adequate cover to the Albanian coast from the north—at a safe distance from any Bulgarian advance in the south in the event of a Bulgarian intervention.) The Allies were also delayed due to protracted though ultimately fruitless secret negotiations to bring Bulgaria into the Allied camp, which would have alleviated Serbia's need for Franco-British help.

In the event, the lack of Allied support sealed the fate of the Serbian army. Against Serbia, the Central Powers marshalled the Bulgarian Army, a German army, and an Austro-Hungarian army, all under the command of Field Marshal August von Mackensen. The Germans and Austro-Hungarians began their attack on 7 October with a massive artillery barrage, followed by attacks across the rivers. Then, on 11 October, the Bulgarian army attacked from two directions, one from the north of Bulgaria towards Niš, the other from the south towards Skopje (see map). The Bulgarian army rapidly broke through the weaker Serbian forces that tried to block its advance. With the Bulgarian breakthrough, the Serbian position became hopeless; their main army in the north faced either encirclement and forced surrender or retreat.

Marshal Putnik ordered a full Serbian retreat, southwards and westwards through Montenegro and into Albania. 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. Marshal Putnik had to be carried around during the entire retreat; he died just over a year later in a French hospital.

The French and British divisions marched north from Thessaloniki in October 1915 under the joint command of French General Maurice Sarrail and British General Bryan Mahon (Commander, British Salonika Force, 1915). However, the London War Office was reluctant to advance too deep into Serbia. So the French divisions advanced up the Vardar river alone. This advance gave some limited help to the retreating Serbian army, as the Bulgarians had to concentrate larger forces on their southern flank to deal with the threat, which led to the Battle of Krivolak (October–November 1915). By the end of November, General Sarrail had to retreat in the face of massive Bulgarian assaults on his positions. During his retreat, the British at Kosturino were also forced to retreat. By 12 December, all Allied forces were back in Greece. The Germans ordered the Bulgarians not to cross the Greek borders, reluctant to risk a Greek entry into the war in response to a Bulgarian invasion in Macedonia. The Allies took advantage of that, reinforcing and consolidating their positions behind the borders.