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.
 Bulgaria, Romania and Greece were content to sit on the fence and observe the fortunes of war before deciding whether to declare their sympathies.

The government of Vasil Radoslavov 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 the UK and France) were all in possession of lands perceived in Bulgaria as Bulgarian.
 Bulgaria, recuperating from the Balkan Wars, sat out the first year of World War I, but 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.
The UK, France, Italy and Russia then declared war on Bulgaria.

Bulgaria, in alliance with Germany, Austria–Hungary and the Ottomans, won military victories against Serbia and Romania, taking much of Macedonia (taking Skopje in October), advancing into Greek Macedonia, and taking Dobruja from the Romanians in September 1916.
However, the war soon became unpopular with the majority of Bulgarian people, who suffered great economic hardship and also disliked fighting their fellow Orthodox Christians in alliance with the Muslim Ottomans.
The Agrarian Party leader, Aleksandar Stamboliyski, was imprisoned for his opposition to the war.
 The Russian Revolution of February 1917 had a great effect in Bulgaria, spreading antiwar and anti-monarchist sentiment among the troops and in the cities.
In June Radoslavov's government resigned.
Mutinies broke out in the army, Stamboliyski was released and a republic was proclaimed.

In September 1918, the French, Serbs, British, Italians and Greeks broke through on the Macedonian front and Tsar Ferdinand was forced to sue for peace.
Stamboliyski favoured democratic reforms, not a revolution.
In order to head off the revolutionaries, he persuaded Ferdinand to abdicate 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 to Greece and part of its Macedonian territory to the new state of the Kingdom of Yugoslavia, and had to give Dobruja back to the Kingdom of Romania (see also Dobruja, Western Outlands, Western Thrace).
Elections in March 1920 gave the Agrarians a large majority, and Stamboliyski formed Bulgaria's first genuinely democratic government.
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.  Bulgaria, Romania and Greece were content to sit on the fence and observe the fortunes of war before deciding whether to declare their sympathies. 
The government of Vasil Radoslavov 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 the UK and France) were all in possession of lands perceived in Bulgaria as Bulgarian.  Bulgaria, recuperating from the Balkan Wars, sat out the first year of World War I, but 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. The UK, France, Italy and Russia then declared war on Bulgaria.
Bulgaria, in alliance with Germany, Austria–Hungary and the Ottomans, won military victories against Serbia and Romania, taking much of Macedonia (taking Skopje in October), advancing into Greek Macedonia, and taking Dobruja from the Romanians in September 1916. However, the war soon became unpopular with the majority of Bulgarian people, who suffered great economic hardship and also disliked fighting their fellow Orthodox Christians in alliance with the Muslim Ottomans. The Agrarian Party leader, Aleksandar Stamboliyski, was imprisoned for his opposition to the war.  The Russian Revolution of February 1917 had a great effect in Bulgaria, spreading antiwar and anti-monarchist sentiment among the troops and in the cities. In June Radoslavov's government resigned. Mutinies broke out in the army, Stamboliyski was released and a republic was proclaimed.
In September 1918, the French, Serbs, British, Italians and Greeks broke through on the Macedonian front and Tsar Ferdinand was forced to sue for peace. Stamboliyski favoured democratic reforms, not a revolution. In order to head off the revolutionaries, he persuaded Ferdinand to abdicate 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 to Greece and part of its Macedonian territory to the new state of the Kingdom of Yugoslavia, and had to give Dobruja back to the Kingdom of Romania (see also Dobruja, Western Outlands, Western Thrace). Elections in March 1920 gave the Agrarians a large majority, and Stamboliyski formed Bulgaria's first genuinely democratic government.
Kingdom of Bulgaria