Which resource is a primary source that could be used for a research paper on the US military’s role in World War I?

A letter written to a family member by an America soldier fighting in the war.

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the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.

honestly i love the walking dead, mainly the quotes. “well, pardon me young man, excuse the [insert bad word here] out of my gosh darn french, but did you just threaten me? ” sksk (of course it's not the exact words)