men vs manpower

men and manpower both are nouns.

Word	Noun	Adjective	Verb	Adverb
men	Yes	No	No	No
manpower	Yes	No	No	No

As nouns, men and manpower are synonyms defined as:

Other synonyms of men include hands, work force, workforce.

men (noun)	manpower (noun)
the force of workers available	the force of workers available