# Declarations of war during World War I

A declaration of war is a formal act by which one nation goes to war against another. A declaration is usually an act of delivering a performative speech (not to be confused with a mere speech) or the presentation of a signed document by an authorized party of a national government in order to create a state of war between two or more sovereign states. In the context of World War I, the official international protocol for declaring war was defined in the Hague Convention of 1907 (or Hague II). For the diplomatic maneuvering behind these events, which led to hostilities between nations, see Diplomatic history of World War I.

## List of war declarations

Below is a table showing the outbreaks of wars between nations which occurred during World War I. Indicated are the dates (during the immediate build-up to, or during the course of, World War I) from which a de facto state of war existed between nations. The table shows both the initiator states and the state at which the declaration of war was aimed. Events listed include those in which there were a simple diplomatic breaking of relations that did not involve any physical attack, as well as those involving overt declarations or acts of aggression.
| Date | Initiator states | Targeted states |
| July 28, 1914 | Austria-Hungary | Serbia |
| August 1, 1914 | German Empire | Russian Empire |
| August 3, 1914 | German Empire | Belgium |
| August 3, 1914 | German Empire | France |
| August 4, 1914 | British Empire | German Empire |
| August 5, 1914 | Montenegro | Austria-Hungary |
| August 6, 1914 | Austria-Hungary | Russian Empire |
| August 6, 1914 | Serbia | German Empire |
| August 8, 1914 | Montenegro | German Empire |
| August 12, 1914 | British Empire | Austria-Hungary |
| August 12, 1914 | France | Austria-Hungary |
| August 23, 1914 | Japan | German Empire |
| August 25, 1914 | Japan | Austria-Hungary |
| August 28, 1914 | Austria-Hungary | Belgium |
| November 1, 1914 | Russian Empire | Ottoman Empire |
| November 5, 1914 | France | Ottoman Empire |
| November 5, 1914 | British Empire | Ottoman Empire |
| November 11, 1914 | Ottoman Empire | Russian Empire |
| November 11, 1914 | Ottoman Empire | Japan |
| November 11, 1914 | Ottoman Empire | British Empire |
| December 2, 1914 | Serbia | Ottoman Empire |
| December 3, 1914 | Montenegro | Ottoman Empire |
| December 5, 1914 | Japan | Ottoman Empire |
| May 23, 1915 | Italy | Austria-Hungary |
| August 21, 1915 | Italy | Ottoman Empire |
| August 28, 1915 | Italy | German Empire |
| October 14, 1915 | Bulgaria | Serbia |
| October 15, 1915 | British Empire | Bulgaria |
| October 15, 1915 | Montenegro | Bulgaria |
| October 16, 1915 | Japan | Bulgaria |
| October 16, 1915 | France | Bulgaria |
| October 19, 1915 | Italy | Bulgaria |
| October 19, 1915 | Russian Empire | Bulgaria |
| March 9, 1916 | German Empire | Portugal |
| March 15, 1916 | Austria-Hungary | Portugal |
| August 28, 1916 | Romania | Austria-Hungary |
| August 28, 1916 | German Empire | Romania |
| August 30, 1916 | Ottoman Empire | Romania |
| September 1, 1916 | Bulgaria | Romania |
| April 6, 1917 | United States | German Empire |
| April 7, 1917 | Panama | German Empire |
| April 7, 1917 | Cuba | German Empire |
| June 27, 1917 | Greece | Austria-Hungary |
| June 27, 1917 | Greece | German Empire |
| June 27, 1917 | Greece | Bulgaria |
| June 27, 1917 | Greece | Ottoman Empire |
| July 22, 1917 | Siam | Austria-Hungary |
| July 22, 1917 | Siam | German Empire |
| August 4, 1917 | Liberia | German Empire |
| August 14, 1917 | China | Austria-Hungary |
| August 14, 1917 | China | German Empire |
| October 26, 1917 | Brazil | German Empire |
| December 7, 1917 | United States | Austria-Hungary |
| December 10, 1917 | Panama | Austria-Hungary |
| April 23, 1918 | Guatemala | German Empire |
| May 6, 1918 | Nicaragua | Austria-Hungary |
| May 6, 1918 | Nicaragua | German Empire |
| May 23, 1918 | Costa Rica | German Empire |
| July 12, 1918 | Haiti | German Empire |
| July 19, 1918 | Honduras | German Empire |