In the aftermath of the Balkan Wars Bulgarian opinion turned against Russia and the western powers, whom the Bulgarians felt had done nothing to help them. The government aligned Bulgaria with Germany and Austria-Hungary, even though this meant also becoming an ally of the Ottomans, Bulgaria's traditional enemy. But Bulgaria now had no claims against the Ottomans, whereas Serbia, Greece and Romania (allies of Britain and France) were all in possession of lands heavily populated by Bulgarians and thus perceived as Bulgarian.
Bulgaria, recuperating from the Balkan Wars, sat out the first year of World War I. When Germany promised to restore the boundaries of the Treaty of San Stefano, Bulgaria, which had the largest army in the Balkans, declared war on Serbia in October 1915. Britain, France and Italy then declared war on Bulgaria.
Although Bulgaria, in alliance with Germany and Austria-Hungary, won military victories against Serbia and Romania, occupying much of Southern Serbia (taking Nish, Serbia's war capital in November 5), advancing into Greek Macedonia, and taking Dobruja from the Romanians in September 1916, the war soon became unpopular with the majority of Bulgarian people, who suffered enormous economic hardship. The Russian Revolution of February 1917 had a significant effect in Bulgaria, spreading antiwar and anti-monarchist sentiment among the troops and in the cities.
In September 1918 the Serbs, British, French, Italians and Greeks broke through on the Macedonian front. While Bulgarian forces stopped them in Dojran and they didn't succeed to occupy Bulgarian lands, Tsar Ferdinand was forced to sue for peace.
In order to head off the revolutionaries, Ferdinand abdicated in favour of his son Boris III. The revolutionaries were suppressed and the army disbanded. Under the Treaty of Neuilly (November 1919), Bulgaria lost its Aegean coastline in favour of the Principal Allied and Associated Powers (transferred later by them to Greece) and nearly all of its Macedonian territory to the new state of Yugoslavia, and had to give Dobruja back to the Romanians (see also Dobruja, Western Outlands, Western Thrace).
The railway from the strategically important port city of Salonika (Thessaloniki) in northern Greece to Belgrade via Skopje (Uskub) offered a direct route to embattled Serbia. In September 1915, with Bulgaria mobilising for an attack on Serbia, Britain and France accepted an offer from the pro-Entente Greek prime minister Eleutherios (Elephferios) Venizelos to land troops at Salonika. The force at Salonika was initially composed of Anglo-French units, many of which had come from Gallipoli. It was reinforced by the Serbian army in exile on Corfu, Italians and a small Russian contingent. It remained until the war’s end.
The force landed on 5 October 1915, the same day that the pro-German Greek king, Constantine I – who was married to the Kaiser’s sister – forced Venizelos to resign. On 6 October 1915, the Central Powers invaded Serbia. Anglo-French units at Salonika pushed north up the Vardar (Axios) river valley to help the Serbs. It was too little, too late. The Serbs retreated through Albania to the Adriatic coast while the Salonika force retired back to the city. Bulgarian and German forces (with some Turkish units) then gathered along the Greek– Serbian and Greek–Bulgarian borders, while the Greek army, supposedly neutral, handed Greece’s Fort Ruppel, which commanded the Struma (Strimón) valley, to the Bulgarians (26 May 1916). In response to these threats, the supreme commander at Salonika, the French general Maurice Sarrail, transformed the city into a fortress surrounded by fieldwork defences. He took full control of the city from the Greeks in mid-1916, establishing the city as an alternative centre for pro-Entente Greek forces and politicians, a policy that embroiled the garrison in internal Greek politics.
While France was keen on the Salonika operation, senior British military advisers to the British government, such as William Robertson, Chief of the Imperial General Staff, were never convinced of the usefulness of peripheral operations such as Salonika, which took troops away from the main western front. By early 1916 the number of British troops at Salonika exceeded 150,000. Robertson vigorously urged a withdrawal from Salonika but political factors made this difficult. The defeat at Gallipoli had lowered Anglo-French prestige in the Balkans and the Entente could ill-afford for Greece to join the Central Powers.
Sarrail’s record of military achievement against the Central Powers was not impressive. On 10 August 1916, Entente troops began preliminary attacks at Lake Dorian (Dojran) before a general autumn 1916 offensive. German- Bulgarian forces pre-empted this with attacks on the western and eastern extremities of Sarrail’s line. In the west, the reconstituted Serbian army in the Flórina sector retreated to Lake Ostrovo (Vegorrítis). Fighting then continued along the Crno (Crna) river east of Flórina. In the eastern sector of the front, the Bulgarians took the Greek town of Serres (Sérrai) in the Struma valley on 25 August, threatening the port of Kavala (Kaválla) whose Greek garrison surrendered without a fight on 14 September 1916. Under pressure, Sarrail put a halt to the faltering offensive at Lake Dorian.
When Sarrail’s troops did finally attack towards Skopje in September 1916, he hoped that this would relieve the hard-pressed Rumanians. Sarrail’s forces took Monastir (Bitola) in the western sector of the front on 19 November 1916 but they advanced no further. As the British struggled up the Struma valley, operations descended into western front-style trench deadlock. Checked, allied troops sat out the winter doing little. By early 1917, Sarrail had some 600,000 men at his disposal – a mixture of French, British, Serbians, Italians and Russians that made command and control difficult. This force was more nominal than real as the unhealthy, swampy climate of Salonika was a breeding ground for diseases such as malaria, paratyphoid and dysentery, which left much of Sarrail’s force in hospital and reduced his fighting strength to about 100,000.
Sarrail attacked again in March 1917. A Franco-Serbian force advanced on a line between Monastir and Lake Prespa, while the British spearheaded an attack at Lake Dorian. Advances were minimal: a few hundred metres were won at the cost of some 15,000 casualties. By the end of May, with Russian units in mutiny, Sarrail called off the offensive, and the front became static. Marie Guillaumat replaced the unpopular Sarrail in December 1917. In July 1918, another French commander, Louis Franchet d’Esperey, replaced Guillaumat. In June 1917, with Venizelos back in power, Greece entered the war (29 June 1917), adding 250,000 men to the Salonika force. On 15 September 1918, at the war’s end, the Salonika force launched the Vardar offensive against weary and ill-equipped Bulgarian opposition. The Bulgarians broke, and by 25 September their retreat had become a rout. The Salonika force advanced deep into enemy territory, reaching the Danube by the armistice. Serbian forces re-occupied Belgrade on 1 November 1918, by which time Bulgaria had surrendered (30 September).
This final victory should not overshadow the fact that the Salonika expedition did very little, except tie up large numbers of Entente troops that could have been used more fruitfully elsewhere. Although battle casualties were low – a reflection of the general inactivity of the front for most of the war – casualties from disease, notably malaria, invalided some 400,000 men. The Germans were right to dub the front ‘the greatest internment camp in the world’.