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.

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

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

And each attack must be judged separately, to decide if it is within the boundaries of a legal act of war.

From New York Times • Dec. 13, 2023

“Are you suggesting I commit an act of war because this man has a hot line to the spirit world?”

From "The Ear, the Eye, and the Arm" by Nancy Farmer

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