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act of war

American  

noun

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

act of war British  

noun

  1. 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.

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

"Hybrid warfare" is when a hostile state carries out an anonymous, deniable attack, usually in highly suspicious circumstances, but stops short of being an attributable act of war.

From BBC • Apr. 9, 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

Denmark would break off relations with the U.S. and insist, with a certain amount of hedging, that this was an act of war, sort of.

From Salon • Apr. 27, 2025

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

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.