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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.

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

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

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

"If I had agreed to their request, then SpaceX would be explicitly complicit in a major act of war and conflict escalation."

From BBC • Sep. 8, 2023

A blockade, like an air strike, was 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

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