The disappointment over the peace treaty after the Balkan wars, caused Bulgaria to enter World War I on the side of the Central Powers. After staying neutral during the first year of the war, the country attacked Serbia in the autumn of 1915. The Serbian army was defeated in a few days, and the Bulgarians advanced through Macedonia towards Salonika which was the Entente bridgehead on Balkan. However, the German supreme command wanted to maintain a front on Balkan to bind Entente troops otherwise available against the Germans on the west front. The Bulgarian advance was thus stopped, and a front line extending from Albania to Thrace was established.
In the autumn of 1916 Romania entered the war on the side of the Entente. The Bulgarian supreme command only had one army group left to engage against the Romanians. However, these forces considered the fighting as a war of liberation for their people in Dobrudja, occupied by Romania only three years before. Both the Romanian army and several Russian supporting divisions were beaten in only two months. Early December the Bulgarian forces supported by German units advanced into Bucurest, the capital of Romania.
A long-lasting embargo from the entente side caused suffering and social unrest in Bulgaria. In September 1918 several Bulgarian divisions in Macedonia deserted and marched towards Sofia. However, the fatigued and poorly organised rebels did not succeed in breaking through the loyal defence forces surrounding the capital, supported by German troops, and the rebellion was put down October 2, 1918.
In the meantime, however, the government had asked for a cease-fire with the Entente. The truce was signed September 29 in Salonika, demanding full retreat of all Bulgarian forces and occupation of strategically vital areas by Entente troops.
Later, in the peace treaty of Neuille November 1919 Bulgaria lost further territories to it's neighbors, including Thrace and the access to the Aegean Sea. Having led the country into two national disasters since 1913, King Ferdinand was forced to abdicate October 3, 1918. His son ascended the throne as Boris III.
(the information has been taken from the site Philatelic Journey through the History of Bulgaria)