act of war

American

noun

an act of aggression by a country against another with which it is nominally at peace.

act of war British

noun

an aggressive act, usually employing military force, which constitutes an immediate threat to peace

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

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act of war

Because a blockade is recognized in international law as an act of war, governments more often characterize their efforts to isolate an adversary as sanctions, embargoes or quarantines that suggest narrower goals.

From The Wall Street Journal ● Apr. 16, 2026

Allies framed the strikes as a law-enforcement action rather than an act of war.

From Salon ● Jan. 3, 2026

After a ceasefire, Modi vowed that "any attack on Indian soil will be considered as an act of war".

From Barron's ● Nov. 17, 2025

A former counter terrorism chief has described how he initially wondered if the poisoning of a former spy and his daughter could have been "an act of war".

From BBC ● Oct. 27, 2024

This was nothing less than an invasion, Khrushchev charged, an act of war.

From "Fallout: Spies, Superbombs, and the Ultimate Cold War Showdown" by Steve Sheinkin

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023