casus belli vs beginning

casus belli and beginning both are nouns.

Word	Noun	Adjective	Verb	Adverb
casus belli	Yes	No	No	No
beginning	Yes	Yes	No	No

As nouns, beginning is a hypernym of casus belli; that is, beginning is a word with a broader meaning than casus belli:

casus belli (noun)	beginning (noun)
an event used to justify starting a war	the act of starting something
	the first part or section of something
	the event consisting of the start of something
	the place where something begins, where it springs into being
	the time at which something is supposed to begin

casus belli (adjective)	beginning (adjective)
	serving to begin