I have long had it assumed Casus Belli was a term used by the Romans (probably because it's Latin), but apparently not so?
Livy uses this term, Livy, History of Rome, Volume XII: Books 40-42 (pg 444 LOEB classics version) edit: though from the comment that is better translated as 'casualties' of war rather than 'cause'.
"viro forti dignum sit, patiatur quodcumque casus belli tulerit, aut victor liberet"
Another example of Livy using roughly this term (but with the meaning I assume the author is looking for) is in Livy Book XXVIII part 7 from this link
"praeuertendum id ratus legatis cum benigno responso dimissis—se neque causam eius belli fuisse nec moram, si modo aequa et honesta condicione liceat, paci facturum"
And later in the same book part 14 (same link as above)
"Romanum Poenumque, quos inter belli causa esset, pari robore animorum armorumque concursuros"
Also see Livy Book XXI (College Series of Latin Authors, Greenough & Peck 1893, pg 40)
"Cathagine acta decreta que forent seque non ducem solum sed etiam causam esse belli"
Seutonius uses a slightly different version that means the same thing, probably having to do with Latin conjugations but it has been 30 years since I last had a Latin class, but in Lives of Caesars part 5 Claudius (pg 48 in the LOEB classics version) he writes:
"ut legatis consularibus simul cum exercitu et triumphalia darentur, ne causam belli quoquo modo quaererent. Aulo Plautio etiam ovationem decrevit ingressoque urbem"
Added correction from Pieter Geerkens: "casus belli" is "the casualties of war" in the nominative case, either singular or plural; and "causam belli" is "the cause of war" in the accusative singular
Casus belli means "an act of war". It does not seem to be used in this sense by any classical Roman writer, but it is a well-established term in modern international law. "Casus" is not the same word as "causa" (cause). The confusion of "casus belli" (act of war) with "causa belli" (cause of war) is a common blunder.