struggle vs join battle

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

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

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

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

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