force back vs push back

force back is not a noun while push back is a noun.

Word	Noun	Adjective	Verb	Adverb
force back	No	No	Yes	No
push back	Yes	No	Yes	No

As verbs, force back and push back are synonyms defined as:

Other synonyms of force back include beat back, drive, repel, repulse.

force back (noun)	push back (noun)
	the act of forcing the enemy to withdraw

force back (verb)	push back (verb)
cause to move back by force or influence	cause to move back by force or influence