The war started in Europe, but soon spread throughout most of the world.
These are many of the countries that fought during the war. The date indicates their declaration of war, or the day they commenced hostilities without a formal declaration.
The Triple Entente or Allies
- Serbia (28 July 1914)
- Russia (1 August 1914)
- France (2 August 1914)
- Great Britain (4 August 1914)
- British Empire Dominions and Colonies (4 August 1914):
- Australia
- India
- Canada
- Newfoundland
- New Zealand
- South Africa
- Japan (23 August 1914)
- Italy (23 May 1915)
- United States (6 April 1917)
These countries also fought with the Allies or declared war on the Central Powers:
- Minor European Powers: Portugal, Belgium, Romania, Greece
- Other countries: Montenegro, San Marino, Brazil, Costa Rica, Cuba, Ecuador, Guatemala, Haiti, Honduras, Nicaragua, Panama, Peru, China, Siam, Liberia
The Central Powers
- Austria-Hungary (28 July 1914)
- Germany (1 August 1914)
- Turkey (5 November 1914)
- Bulgaria (14 October 1915)
These were the main European countries to remain neutral throughout the war:
- The Netherlands
- Switzerland
- Spain
- Sweden
- Norway
- Denmark
- Finland
- Iceland