foot soldier vs infantryman

foot soldier and infantryman both are nouns.

Word	Noun	Adjective	Verb	Adverb
foot soldier	Yes	No	No	No
infantryman	Yes	No	No	No

As nouns, foot soldier and infantryman are synonyms defined as:

Other synonyms of foot soldier include footslogger, marcher.

foot soldier (noun)	infantryman (noun)
fights on foot with small arms	fights on foot with small arms
an assistant subject to the authority or control of another