The outbreak or start of a war; the act or moment when hostilities begin (often used as '開戦する').
政府は隣国との開戦を公式に発表した。
The government officially announced the outbreak of war with the neighboring country.
開戦の報は市民に大きな衝撃を与えた。
The news of the outbreak of war gave the citizens a great shock.
歴史の授業で第一次世界大戦の開戦経緯を学んだ。
In history class we studied the circumstances of the outbreak of World War I.