noun
act of war; hostile act
Refers to a military action that constitutes an act of war under international law or in a broader conflict context.
その攻撃は戦争行為とみなされた。
The attack was regarded as an act of war.
国連はその侵攻を戦争行為と認定した。
The UN determined the invasion to be an act of war.
Compound of 戦争 (sensō, 'war') + 行為 (kōi, 'act, deed').