Shouldn't that be casus belli? (genitive, not nominative) <pedant>Shouldn't that be casum belli with the cause in the accusative as it is the object of the verb to provoke?<\pedant>
Hmmm
Casus (Case) and Bellum (War) are both nouns so we are talking about declensions. The "Case for War" translates as Casus (Nominative, or Subject) Belli (Genitive, the case which indicates possession or association) The correct term, is therefore, casus belli. If you don't believe me look it up in a dictionary. I knew that phrase parsing for 'O' Level Latin would eventually have a use!