fight vs join battle

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

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

As verbs, join battle is a hyponym of fight; that is, join battle is a word with a more specific, narrower meaning than fight:

fight (noun)	join battle (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

fight (verb)	join battle (verb)
be engaged in a fight; carry on a fight	engage in a conflict
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