This is a list of countries that participated in World War I, sorted by alphabetical order.
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.
| Date | Declarer | On |
|---|
| 1914 |
|---|
| July 28 | Austria-Hungary}} | Kingdom of Serbia|name=Serbia}} |
| August 1 | German Empire|name=Germany}} | Russian Empire|name=Russia}} |
| August 3 | German Empire|name=Germany}} | French Third Republic|name=France}} |
| August 4 | German Empire|name=Germany}} | Belgium|state}} |
| United Kingdom}} | German Empire|name=Germany}} |
| August 5 | Kingdom of Montenegro|name=Montenegro}} | Austria-Hungary}} |
| August 6 | Austria-Hungary}} | Russian Empire|name=Russia}} |
| Kingdom of Serbia|name=Serbia}} | German Empire|name=Germany}} |
| August 9 | Kingdom of Montenegro|name=Montenegro}} | German Empire|name=Germany}} |
| August 11 | French Third Republic|name=France}} | Austria-Hungary}} |
| August 12 | United Kingdom}} | Austria-Hungary}} |
| August 22 | Austria-Hungary}} | Belgium|state}} |
| August 23 | Empire of Japan|name=Japan}} | German Empire|name=Germany}} |
| August 25 | Empire of Japan|name=Japan}} | Austria-Hungary}} |
| October 5 | Principality of Albania|name=Albania}} | Austria-Hungary}} |
| November 1 | Russian Empire|name=Russia}} | Ottoman Empire}} |
| November 2 | Kingdom of Serbia|name=Serbia}} | Ottoman Empire}} |
| November 3 | Kingdom of Montenegro|name=Montenegro}} | Ottoman Empire}} |
| November 5 | United Kingdom}}
{{flagu|French Third Republic|name=France}} | Ottoman Empire}} |
| 1915 |
|---|
| May 23 | Kingdom of Italy|name=Italy}} | Austria-Hungary}} |
| August 21 | Kingdom of Italy|name=Italy}} | Ottoman Empire}} |
| October 14 | Kingdom of Bulgaria|name=Bulgaria}} | Kingdom of Serbia|name=Serbia}} |
| October 15 | United Kingdom}}
{{flagu|Kingdom of Montenegro|name=Montenegro}} | Kingdom of Bulgaria|name=Bulgaria}} |
| October 16 | French Third Republic|name=France}} | Kingdom of Bulgaria|name=Bulgaria}} |
| October 19 | Kingdom of Italy|name=Italy}}
{{flagu|Russian Empire|name=Russia}} | Kingdom of Bulgaria|name=Bulgaria}} |
| 1916 |
|---|
| March 9 | German Empire|name=Germany}} | Portugal}} |
| March 15 | Austria-Hungary}} | Portugal}} |
| August 27 | Kingdom of Romania|name=Romania}} | Austria-Hungary}} |
| Kingdom of Italy|name=Italy}} | German Empire|name=Germany}} |
| August 28 | German Empire|name=Germany}} | Kingdom of Romania|name=Romania}} |
| August 30 | Ottoman Empire}} | Kingdom of Romania|name=Romania}} |
| November 1 | Kingdom of Bulgaria|name=Bulgaria}} | Kingdom of Romania|name=Romania}} |
| 1917 |
|---|
| April 6 | United States|1912}} | German Empire|name=Germany}} |
| April 7 | Cuba}}
{{flagu|Panama}} | German Empire|name=Germany}} |
| April 10 | Kingdom of Bulgaria|name=Bulgaria}} | United States|1912}} |
| April 13 | Bolivia|state}} | German Empire|name=Germany}} |
| April 20 | Ottoman Empire}} | United States|1912}} |
| July 2 | Kingdom of Greece|name=Greece}} | German Empire|name=Germany}}
{{flagu|Austria-Hungary}}
{{flagu|Ottoman Empire}}
{{flagu|Kingdom of Bulgaria|name=Bulgaria}} |
| July 22 | Thailand|name=Siam}} | German Empire|name=Germany}}
{{flagu|Austria-Hungary}}
{{flagu|Ottoman Empire}} |
| August 4 | Liberia}} | German Empire|name=Germany}} |
| August 14 | Republic of China (1912–1949)|1912|name=China}} | German Empire|name=Germany}}
{{flagu|Austria-Hungary}} |
| October 6 | Peru|1825}} | German Empire|name=Germany}} |
| October 7 | Uruguay}} | German Empire|name=Germany}} |
| October 26 | Brazil|1889}} | German Empire|name=Germany}} |
| December 7 | United States|1912}} | Austria-Hungary}} |
| December 7 | Ecuador}} | German Empire|name=Germany}} |
| December 10 | Panama}} | Austria-Hungary}} |
| December 16 | Cuba}} | Austria-Hungary}} |
| 1918 |
|---|
| April 23 | Guatemala}} | German Empire|name=Germany}} |
| May 8 | Nicaragua}} | German Empire|name=Germany}}
{{flagu|Austria-Hungary}} |
| May 23 | Costa Rica|state}} | German Empire|name=Germany}} |
| July 12 | Republic of Haiti (1859–1957)|name=Haiti}} | German Empire|name=Germany}} |
| July 19 | Honduras}} | German Empire|name=Germany}} |
| November 10 | Kingdom of Romania|name=Romania}} | German Empire|name=Germany}} |
1. ^{{cite web|url=http://www.journal.forces.gc.ca/vo8/no3/moffat-eng.asp|title=Forgotten Battlefields – Canadians in Siberia 1918 – 1919|work=Canadian Military Journal|last=Moffat|first=Ian C. D|accessdate=2018-11-18|publisher=Department of National Defence}}
2. ^{{Cite web|url=https://www.loc.gov/law/help/digitized-books/world-war-i-declarations/ww1-gazettes/Japan-1-OCR-SPLIT.pdf|title=宣戦の詔書 [Sensen no shōsho, Imperial Rescript on Declaration of War] Kanpō, Extra ed., Aug. 23, 1914.|last=|first=|date=23 August 1914|website=|archive-url=|archive-date=|dead-url=|access-date=|language=Japanese}}
3. ^{{cite book|last=Fox|first=Sir Frank|authorlink=Frank Fox (author)|title=The Agony of Belgium The Invasion of Belgium in WWI August-December 1914|url=http://sirfrankfox.com/excerpt-from-the-agony-of-belgium/|year=1914|publisher=Beaumont Fox 2nd edition 2014|page=19}}
4. ^{{cite web|url=https://timesmachine.nytimes.com/timesmachine/1903/09/13/102434270.pdf|title=Awaiting a Crisis in Belgium|website=New York Times|date=13 September 1903|accessdate=16 November 2018}}
5. ^{{cite web|url=http://net.lib.byu.edu/~rdh7/wwi/versa/versa2.html|title=Peace Treaty of Versailles Articles 31 - 117 and Annexes Political Clauses for Europe|website=net.lib.byu|date=11 November 1998|accessdate=16 November 2018}}