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

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

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

Since a blockade is an act of war against an enemy state, China would call its action something else, such as a quarantine.

From The Wall Street Journal • Oct. 8, 2025

"The incident was far more serious than originally portrayed and could have amounted to an act of war," the New York Times reported.

From BBC • Sep. 14, 2023

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

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

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