The Great War was the largest war to that point in human history. Who were the Allies in World War I?
At the outbreak of World War I, Britain, France, and Russia entered the war on the side of Serbia, forming the Triple Entente. They were joined by Belgium, Italy, the United States, and a number of other countries. They faced the Central Powers of Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria.

For more on the Triple Entente and their allies during World War I, read on.
Triple Entente
Following the assassination of Archduke Franz Ferdinand by Gavrilo Princip on June 28, 1914, Austria-Hungary invaded Serbia exactly a month later on July 28. The invasion of Serbia was the catalyst for numerous other nations joining the war, which soon became the first truly global war in history.
Entry into the war was motivated by different goals for each of the major powers. In some cases, countries were simply honoring existing alliances and had no true ill will toward their adversaries.
Within weeks, Germany had invaded Luxembourg and Belgium, France had invaded Alsace, and the British had sent troops to support the French.
The composition of World War I was, in a sense, a battle between old and young nations. Both Germany and Austria-Hungary were relatively new global powers, each formed in the 1800s.
The German states had officially unified in 1871, four years after the Austro-Hungarian Compromise in 1867.
In contrast, Britain, France, and Russia had aggressively expanded their territories for centuries through overseas colonialism and, in Russia’s case, annexing neighboring land.
Germany was keen to expand its own borders. Britain and other major powers opposed this, despite having created a vast empire over the previous centuries. Tensions had been building between the European powers for decades when the Archduke was assassinated.
Germany and Austria-Hungary would need to fight a war on two fronts. Britain, France, and Russia formed the Triple Entente, an alliance dedicated to fighting Germany and Austria.
Other Participants
Before the war, Germany had also formed an alliance with the Ottoman Empire. The Ottoman Empire had been in decline since the Industrial Revolution and its new leaders saw the war as an opportunity to regain some of its lost territories.
Many other countries joined the fight. In Europe, Belgium joined the war on the side of Britain and France. Several of the Balkan states joined the war at the side of Russia, feeling threatened by German and Austrian expansionism.
There were a number of German colonies in Asia. Japan, which was also looking to expand its territory, aligned itself with the Allies by invading German territory in China.
Italy remained neutral for the first year of the war. In 1915, however, it joined on the side of the Allies. Germany and Austria had already been fighting a war on two fronts but now they also had to face the threat of Italy in the south.
Shifting Tide
Tensions between the United States and Germany built throughout the war. America took a stance of neutrality but Britain was an important trading partner. Germany announced that it would sink any ship entering British waters and these actions resulted in the deaths of hundreds of American citizens.
When an American ship, the “SS Housatonic”, was sunk by a U-boat in 1917, the United States declared war on Germany. The Western Front had been locked in a violent stalemate for years, resulting in millions of deaths.
At the height of the war, the Allies included Serbia, Russia, Britain, France, Belgium, Italy, and the United States of America. Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria became known as the Central Powers.
The influx of American troops helped turn the tide of the war in favor of the Allies. Meanwhile, the Eastern Front underwent changes that would shape the 20th Century.
Germany helped the exiled Vladimir Lenin reenter Russia in the hope that his arrival would spark a revolution and end the war. The plan came to fruition and Tsar Nicholas II was overthrown, ending centuries of rule by the Russian monarchy. The new Soviet Russia soon negotiated its peace with Germany.
Though Germany no longer had to contend with the might of Russia, it was losing the war on the Western Front. On November 11, 1918, the Armistice was signed, ending the war in Europe without the need for further bloodshed. For Germany, it was a surrender in all but its name.