The military alliance that fought against the Central Powers was known as the Allies. Initially this alliance was based around the four great powers of Russia, France, Japan and the British Empire, along with the smaller states of Serbia, Montenegro and Belgium that also went to war in 1914.
Italy joined the Allies in May 1915 by declaring war on its neighbour, Austria-Hungary. This opened up a new battlefront – the Italian Front – along their common border that would be the main focus of Italian military operations for the rest of the war. Italy did not declare war on Germany until August 1916. The Italian Front, like the Western Front, was characterised by bloody trench warfare. Conditions were made even worse by the fact that much of it was fought at high altitude in the Alps.
In 1916 Romania and Portugal joined the war on the Allies' side, but the former was soon invaded by the Central Powers, which occupied nearly all of its territory. Similar fates had been suffered by Belgium in 1914, Serbia in late 1915 and Montenegro in early 1916. With the exception of Montenegro these countries stayed in the war, establishing governments-in-exile and maintaining armies in the field.
The most dramatic upheaval occurred in 1917, when the outbreak of revolution in Russia led to its withdrawal from the alliance and the signing of a seperate peace with the Central Powers, the Treaty of Brest-Litovsk, in early 1918. Compensating for the loss of this great power to the Allies was the addition of the United States of America, which declared war on Germany in April 1917. But American troops arrived in France agonisingly slowly, and it was not until late 1918 that they began to make a real impact at the front line.
The American action led to a flurry of like-minded declarations of war by the Republic of Liberia and eight Central and South American states, including Brazil, Cuba and Haiti. Their membership of the alliance – like China's declaration of war on Germany in August 1917 – was more symbolic than substantive. China’s decision was seen as an essential step in winning back international recognition and respect for China’s sovereignty from the Western powers and Japan. The Chinese Republican leadership placed much hope in the Allied promise that victory would be followed by a dramatic rearrangement of the international order that would address past injustices.
Greece also formally joined the Allies in June 1917, but in very controversial circumstances and under intense pressure from the French and British governments. An Allied army of French, British, Serbian and Italian troops had, with Greek permission, occupied the Greek port of Salonika to fight the Bulgarians in northern Greece and southern Serbia since late 1915. From then on pro-Allied Greek political factions had agitated for the country to openly join the Allies. The decision to do so came only after the pro-German King of Greece, Constantine I, was forced to abdicate to stop the issue leading to civil war. Subsequently the Greek Army was wholeheartedly committed to the Allied cause, with Greek troops playing a leading role in the final offensives against the Bulgarians in 1918.