bellum

bellum, belli is a second declension NEUTER noun: bellum, belli, bello, bellum, bello, plural bella, bellorum, bellis. Because it is neuter, bellum is both nominative and accusative and bella is both nominative and accusative plural.

The verb it takes is gero, not facio: bellum gerere, "to wage war", and bello confecto, "the war having been finished", is Caesar's standard ablative absolute for wrapping up a campaign. The locative-looking belli, "in wartime", pairs with domi in domi bellique, at home and at war. The older form of the word was duellum, which is where the English doublet "duel" comes from, and that d is also why bellum has nothing to do with bellus, "pretty" (the ancestor of "beautiful"). Watch the homograph: bella can be wars or pretty things, and only context tells you which.