During World War I (1914-1918), the major alliances were known as the Allies and the Central Powers.
Allies: The primary members of the Allies included:
- France
- United Kingdom
- Russia (until 1917, when they withdrew due to the Russian Revolution)
- Italy (joined the Allies in 1915, having initially been part of the Triple Alliance with Germany and Austria-Hungary)
- United States (joined in 1917)
- Japan
- Belgium
- Serbia
- Greece
- Romania
- Portugal
- Montenegro
- Brazil (joined in 1917)
There were also other countries that supported the Allies, but these were the main participants.
Central Powers: The primary members of the Central Powers were:
- Germany
- Austria-Hungary
- Ottoman Empire (modern-day Turkey and surrounding regions)
- Bulgaria
These two alliances faced each other throughout the conflict, which resulted in a widespread and devastating war across multiple fronts.