join battle vs fight

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

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

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

Other hypernyms of join battle include contend, struggle.

join battle (noun)	fight (noun)
	a hostile meeting of opposing military forces in the course of a war
	the act of fighting; any contest or struggle
	a boxing or wrestling match
	an aggressive willingness to compete
	an intense verbal dispute

join battle (verb)	fight (verb)
engage in a conflict	be engaged in a fight; carry on a fight
	fight against or resist strongly
	make a strenuous or labored effort
	exert oneself continuously, vigorously, or obtrusively to gain an end or engage in a crusade for a certain cause or person; be an advocate for