> Since 1945, developments in international law such as the United Nations Charter, which prohibits both the threat and the use of force in international conflicts, have made declarations of war largely obsolete in international relations

> Declarations of war have been exceedingly rare since the end of World War II.

> In his study Hostilities without Declaration of War (1883), the British scholar John Frederick Maurice showed that between 1700 and 1870 war was declared in only 10 cases, while in another 107 cases war was waged without such declaration

Most of these statements have source references.

While this doesn't cover the whole history, you are welcome to bring up sources contradicting my statement.