war vs go to war

war is a noun but go to war is not a noun.

Word	Noun	Adjective	Verb	Adverb
war	Yes	No	Yes	No
go to war	No	No	Yes	No

As verbs, go to war is a hyponym of war; that is, go to war is a word with a more specific, narrower meaning than war:

Other hyponyms of war include blitzkrieg, take arms, take up arms.

war (noun)	go to war (noun)
the waging of armed conflict against an enemy
a concerted campaign to end something that is injurious
an active struggle between competing entities
a legal state created by a declaration of war and ended by official declaration during which the international rules of war apply

war (verb)	go to war (verb)
make or wage war	commence hostilities