Participants in World War I
This is a list of countries that participated in World War I, sorted by alphabetical order.
The Entente Powers 
Note: The "Entente Powers" are also known as the Allies of World War I or the Allies.
- Belgium (Belgian colonial empire)
- Costa Rica
- Czechoslovak Legions
- France (French colonial empire)
- San Marino
- Siam (now Thailand)
- United Kingdom (British Empire)
- United States
The Central Powers 
- Austria-Hungary (now Austria, Bosnia and Herzegovina, Croatia, Czech Republic, Hungary, parts of northeast Italy, partially Poland, Romania, Slovakia, Slovenia and Ukraine)
- Azerbaijan (now Azerbaijan and Eastern Armenia)
- Bulgaria (now Bulgaria, partially Greece, partially Republic of Macedonia)
- Dervish State (now parts of Somalia)
- Germany (German colonial empire, at the time the empire occupied Burundi, Cameroon, small parts of China, partially Gabon, Germany, Ghana, Marshall Islands, Federated States of Micronesia, Namibia, northeastern Nigeria, Palau, northern Papua New Guinea, partially Poland, Republic of the Congo, Rwanda, Samoa, mainland Tanzania, and Togo)
- Jabal Shammar (now most of Saudi Arabia, parts of Iraq and Jordan)
- Ottoman Empire (now Iraq, Israel, Jordan, Lebanon, partially Saudi Arabia, Syria, most of Turkey, and other territories it lost before the war.)
Neutral states 
- Afghanistan - received a German diplomatic mission trying to convince it to act against the British in India
- Denmark - Traded with both sides.
- El Salvador
- Ethiopia - received a German diplomatic mission trying to convince it to act against Italy, United Kingdom and France in East Africa
- Liechtenstein - Had a customs and monetary union with Austria-Hungary.
- Luxembourg - Never declared war on the Central Powers despite being invaded and occupied by Germany.
- Mexico - Declined an alliance with Germany (see Zimmermann Telegram).
- Netherlands - An ally of the United Kingdom by treaty. Traded with both sides.
- Norway - Gave naval assistance to the United Kingdom.
- Persia - Occupied by Turk, British and Russian troops.
- Spain - Also treaty bound ally to the United Kingdom.
- Sweden - Non-belligerent.
- Switzerland - Switzerland did declare a "state of siege".
- Venezuela - Supplied the Allies with oil.
Declarations of War 
The following table shows the timeline of the several declarations of war among the belligerent powers. Entries on a yellow background show severed diplomatic relations only, not actual declarations of war.
- ^ The British Empire was the common term for the United Kingdom and its overseas possessions at this time. The term refers to the colonies it controlled, to the Indian Empire, which, while not officially designated a colony, was functionally indistinguishable from one, and the British Dominions - former colonies which were self-governing in most respects but whose foreign policy was controlled by London; Australia, Canada, Newfoundland, New Zealand and South Africa. At that time, the United Kingdom of Great Britain and Ireland also included the entire island of Ireland.