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).
[1] For the diplomatic maneuvering behind these events, which led to hostilities between nations, see Diplomatic history of 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.
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.