If I'm not terribly mistaken, pronouncing it as bell-eye would make it plural - wars instead of war. Bellum = War, Bellae (sp?, "eye") = wars, Belli = for war. In this case pluralizing it makes it somewhat ungrammatical, in the sense that casus belli, taken together, is a compound noun, and the "casus" would only ever be pluralized - thus "casae belli", as in "courts martial" or "governors general".