casus belli
noun
ca·sus bel·li ˈkä-səs-ˈbe-ˌlē 
ˈkā-səs-ˈbe-ˌlī
ˈkā-səs-ˈbe-ˌlī plural casus belli
: an event or action that justifies or allegedly justifies a war or conflict
Example Sentences
Recent Examples on the Web By exposing this plan, the Biden administration sought to undermine its emotional power and stop the Kremlin from manufacturing a casus belli, or justification for war. 
Scott Radnitz, The Conversation, 17 Feb. 2022 India was itching to intervene militarily but lacked a cast-iron casus belli. Tunku Varadarajan, WSJ, 18 Mar. 2022 The Tonkin Gulf incident, which provided a casus belli for the Vietnam War, gets a single sentence. Fred O’brien, National Review, 3 Mar. 2022 But invocations of genocide represent more than just a superficial casus belli. New York Times, 19 Feb. 2022 Russia had all but warned that this would be casus belli. Mario Loyola, National Review, 17 Feb. 2022 Also on Wednesday, a senior Biden administration official, who spoke on condition of anonymity, accused Moscow of trotting out a bevy of false claims with the intention of using them as a casus belli for an incursion into Ukraine. Los Angeles Times, 17 Feb. 2022 False flag attacks are risky, while leaders seeking to manufacture a casus belli can select from a range of subtler and less costly alternatives. Scott Radnitz, The Conversation, 17 Feb. 2022 Analysts say a purely political casus belli could also arise, such as a Russian claim that American, British and other NATO countries are providing weapons to Ukraine that pose a risk to Russian security. New York Times, 25 Jan. 2022 See More
Scott Radnitz, The Conversation, 17 Feb. 2022 India was itching to intervene militarily but lacked a cast-iron casus belli. Tunku Varadarajan, WSJ, 18 Mar. 2022 The Tonkin Gulf incident, which provided a casus belli for the Vietnam War, gets a single sentence. Fred O’brien, National Review, 3 Mar. 2022 But invocations of genocide represent more than just a superficial casus belli. New York Times, 19 Feb. 2022 Russia had all but warned that this would be casus belli. Mario Loyola, National Review, 17 Feb. 2022 Also on Wednesday, a senior Biden administration official, who spoke on condition of anonymity, accused Moscow of trotting out a bevy of false claims with the intention of using them as a casus belli for an incursion into Ukraine. Los Angeles Times, 17 Feb. 2022 False flag attacks are risky, while leaders seeking to manufacture a casus belli can select from a range of subtler and less costly alternatives. Scott Radnitz, The Conversation, 17 Feb. 2022 Analysts say a purely political casus belli could also arise, such as a Russian claim that American, British and other NATO countries are providing weapons to Ukraine that pose a risk to Russian security. New York Times, 25 Jan. 2022 See More Word History
Etymology
New Latin, occasion of war
First Known Use
circa 1841, in the meaning defined above