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

But authorities there refused to extradite him and let him go, judging that any such Ukrainian attack would have been a legitimate act of war rather than a crime.

From Barron's ● Aug. 20, 2026

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

From Salon ● Jan. 3, 2026

In the end, it was an act of war that set the stage for an ambitious plan to end the conflict once and for all.

From The Wall Street Journal ● Oct. 5, 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

German newspapers defended the sinking as a justified act of war.

From "The War to End All Wars: World War I" by Russell Freedman

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