join battle vs struggle

join battle is not a noun while struggle is a noun.

Word	Noun	Adjective	Verb	Adverb
join battle	No	No	Yes	No
struggle	Yes	No	Yes	No

As verbs, struggle is a hypernym of join battle; that is, struggle is a word with a broader meaning than join battle:

Other hypernyms of join battle include contend, fight.

join battle (noun)	struggle (noun)
	strenuous effort
	an energetic attempt to achieve something
	an open clash between two opposing groups (or individuals)

join battle (verb)	struggle (verb)
engage in a conflict	be engaged in a fight; carry on a fight
	to exert strenuous effort against opposition
	climb awkwardly, as if by scrambling
	make a strenuous or labored effort