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Synonyms

casus belli

American  
[key-suhs bel-ahy, bel-ee, kah-soos bel-lee] / ˈkeɪ səs ˈbɛl aɪ, ˈbɛl i, ˈkɑ sʊs ˈbɛl li /

noun

plural

casus belli

  1. an event or political occurrence that brings about a declaration of war.

casus belli British  
/ ˈkɑːsʊs ˈbɛliː /

noun

  1. an event or act used to justify a war

  2. the immediate cause of a quarrel

"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

Etymology

Origin of casus belli

1840–50; < New Latin: literally, occurrence of war

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.

In his speech announcing the war, Mr. Trump explained the extensive casus belli to the American people.

From The Wall Street Journal • Mar. 2, 2026

Rather, this narrow goal may be intended to serve as an official casus belli, giving Russian diplomats something to cite, however implausible, particularly at the United Nations.

From New York Times • Feb. 24, 2022

The comments prompted alarm among U.S. officials who feared he was once again formulating a casus belli ahead of possible new military action against Ukraine.

From Washington Post • Feb. 3, 2022

In such a febrile atmosphere, the risk of a misunderstanding or unplanned escalation is greater, and Russia could use such an incident as a casus belli.

From Reuters • Jan. 14, 2022

Others attacked this conclusion, and the skeletons became one of the casus belli of the evolution wars.

From "1491" by Charles C. Mann

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.