A law passed in 1901 made all male Serbians aged between 21 and 46 liable for compulsory military service. By 1912 the system provided an army of about 260,000 men. This was about 10% of the adult population.
In 1912 Serbia joined with Greece, Bulgaria and Montenegro to form the Balkan League. In October 1912 the Balkan League armies captured most of the Turkish territory in Europe. The conflict was brought to an end by the signing of the Treaty of London in May, 1913.
On the outbreak of the First World War, Serbia had an army of 360,000 men. During 1914, the Serbian Army resisted three successive Austro-Hungarian offensives. However, it virtually exhausted the Army's manpower and it was forced to recruit men over sixty. The army also accepted women, including the British nurse, Flora Sandes.
Serbia pleaded for help and eventually in September 1915, Britain and France accepted the invitation from the Greek prime minister, Eleutherios Venizelos, to land Allied troops at Salonika, a strategically important Greek port on the Aegean coast of Macedonia. As there was a direct railway link between Salonika and Belgrade, this became the best route to send Allied aid to Serbia.
The first Anglo-French troops arrived at Salonika on 5th October, 1915. With Bulgarian and German troops on the frontier, the French commander, General Maurice Sarrail and General George Milne, the leader of the British troops, turned Salonika and its surrounds into an entrenched zone. This included a trench-system similar to the one on the Western Front.
The arrival of Allied troops in Macedonia failed to stop the advance of the Central Powers in Serbia. Overwhelmed by the joint Austro-German and Bulgarian invasion in October 1915, the Serbian Army was forced to retreat to the Albanian mountains. By January 1916, over 155,000 Serbian soldiers and civilians had been evacuated to Corfu.
After recuperation, over 80,000 Serbian troops were sent to Salonika. Considered to be the most aggressive of all the allied troops, the Serbian Army took part in the victory over the Bulgarian Army at the Vardar Offensive in September 1918.
It is estimated that the Serbian Army suffered about 125,000 deaths during the First World War. About 65 per cent were due to sickness, especially the typhus epidemic that had taken place in Serbian trenches during the autumn of 1915.