Declaration of war — an official statement initiating war; commonly appears in the phrase '宣戦布告'.
政府は隣国に対して宣戦布告を行った。
The government declared war on the neighboring country.
歴史の授業で第一次世界大戦の宣戦の経緯を学んだ。
In history class we studied the circumstances of the declarations of war in World War I.