major power vs res publica

major power and res publica both are nouns.

Word	Noun	Adjective	Verb	Adverb
major power	Yes	No	No	No
res publica	Yes	No	No	No

As nouns, res publica is a hypernym of major power; that is, res publica is a word with a broader meaning than major power:

major power: a state powerful enough to influence events throughout the world
res publica: a politically organized body of people under a single government

Other hypernyms of major power include body politic, commonwealth, country, land, nation, state.

major power (noun)	res publica (noun)
a state powerful enough to influence events throughout the world	a politically organized body of people under a single government