trench warfare vs armed combat

trench warfare and armed combat both are nouns.

Word	Noun	Adjective	Verb	Adverb
trench warfare	Yes	No	No	No
armed combat	Yes	No	No	No

As nouns, armed combat is a hypernym of trench warfare; that is, armed combat is a word with a broader meaning than trench warfare:

trench warfare: a type of armed combat in which the opposing troops fight from trenches that face each other
armed combat: an engagement fought between two military forces

Other hypernyms of trench warfare include combat.

trench warfare (noun)	armed combat (noun)
a type of armed combat in which the opposing troops fight from trenches that face each other	an engagement fought between two military forces
a struggle (usually prolonged) between competing entities in which neither side is able to win