A formal announcement by a state proclaiming that it is at war with another state; the declaration of war.
第二次世界大戦では各国の宣戦布告が相次いだ。
During World War II, declarations of war by various countries followed one after another.
国会は宣戦布告を受けて緊急会議を開いた。
Parliament held an emergency session in response to the declaration of war.
近代国際法では宣戦布告の手続きや効果について議論がある。
In modern international law there are debates about the procedures and effects of declarations of war.