casus

Casus, casus, m., is fourth declension and is simply the noun of cado - a falling. From there: the fall of a city, an accident, a chance event, and then a misfortune, since the falls people write about are rarely lucky ones. Genitive singular casus is spelled like the nominative, and the plural casus like both.

The grammarians borrowed the same metaphor. A casus is a way a noun falls away from the upright nominative, the casus rectus, and English took "case" for the grammar, the lawsuit and the suitcase alike. The ablative casu is the ordinary adverb "by accident", and its near twin is forte. Casus belli is late Latin, not Cicero, but it is built correctly.