Declarations of war during World War II
This is a timeline of declarations of war during World War II.
A declaration of war is a formal act by which one nation goes to war against another. The declaration is usually the act of delivering a performative 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. The official international protocol for declaring war was defined in The Hague Peace Conference of 1907. For the diplomatic maneuvering behind these events, which led to hostilities between nations during World War II, see Diplomatic history of [World War II].
List of war declarations
Indicated in the table are the dates, from which a de facto state of war existed between nations. The table shows the "Initiator Nation" and the nation at which the aggression was aimed, or "Targeted Nation". Events listed include those in which there were simple diplomatic breaking of relations that did not involve hostilities, as well as those involving overt declarations or acts of aggression. In rare cases, war between two nations occurred twice, with an intermittent period of peace. The list here does not include peace treaties or periods of any armistice.| Date | Initiator nation | Targeted nation | Type | Notes/comments | Document/event |
| 1939-09-01 |  |