treaty vs written agreement

treaty and written agreement both are nouns.

Word	Noun	Adjective	Verb	Adverb
treaty	Yes	No	No	No
written agreement	Yes	No	No	No

As nouns, written agreement is a hypernym of treaty; that is, written agreement is a word with a broader meaning than treaty:

treaty: a written agreement between two states or sovereigns
written agreement: a legal document summarizing the agreement between parties

treaty (noun)	written agreement (noun)
a written agreement between two states or sovereigns	a legal document summarizing the agreement between parties