1915 a secret treaty of
alliance was signed by Bulgaria and Germany, containing a clause that
promised Serbian, Greek, and Romanian territories to the Bulgarians.
Thus the quadripartite alliance was born, composed of Germany,
Austria-Hungary, Turkey, and Bulgaria.

By September 1915 Bulgarian troops were mobilized and began to deploy
along the borders of Greece and Serbia. On October 1, 1915, Bulgaria
declared war on Serbia and, with the assistance of Austrian and German
troops, succeeded in defeating the Serbian army. At the same time the
Bulgarian army began to advance on Macedonia. There the local
population, a proportion of which was openly sympathetic to Bulgarian
aspirations, joined in the fighting on the side of the Bulgarians.
Although the Bulgarian army attempted to drive the entente forces from
southern Macedonia, it met with failure. This defeat was followed by a
period of prolonged trench warfare on the Balkan front. By 1916 Bulgaria
was also at war with Romania and, with the help of German and Austrian
units, managed a victory over the Romanians.

While the war dragged on, the internal political situation was rapidly
deteriorating. The country was in a state of economic chaos, and the
living conditions of laborers and peasants continued to decline. Farm
production dropped quickly, resulting in famine and soaring prices.
These dire conditions gave a strong impetus to the growing antiwar
movement in the country. The movement was headed by the left-wing
Socialists, who attempted to correlate the antiwar movement with
socialist propaganda. The Russian Revolution of 1917 stirred some
elements of the Bulgarian population who, like the Russian people, felt
that their government failed to represent their interests and was
unresponsive to their needs. There were open revolts in the towns and
villages; underground activities were growing within the Bulgarian army
itself.

By 1918 Bulgaria and the Central Powers were defeated, leaving Bulgaria
in a worse position than before the war. Hopes of regaining Thrace and
Macedonia were dashed, and the country was immeasurably weakened by
external fighting and internal division. The people were frustrated and
bitter. Although the war had stimulated Bulgaria's industry--there were
345 industrial enterprises in 1911 and 1,404 in 1924--it had been costly
in other respects. Bulgaria was forced to pay both reparations and
payments for the allied occupation that followed. Taxes