rearguard vs detachment

rearguard and detachment both are nouns.

Word	Noun	Adjective	Verb	Adverb
rearguard	Yes	No	No	No
detachment	Yes	No	No	No

As nouns, detachment is a hypernym of rearguard; that is, detachment is a word with a broader meaning than rearguard:

rearguard: a detachment assigned to protect the rear of a (retreating) military body
detachment: a small unit of troops of special composition

rearguard (noun)	detachment (noun)
a detachment assigned to protect the rear of a (retreating) military body	the act of releasing from an attachment or connection
	coming apart
	avoiding emotional involvement
	a small unit of troops of special composition
	the state of being isolated or detached