Final answer:
A letter written to a family member by an American soldier fighting in the war is a primary source for researching the US military's role in World War I.
Step-by-step explanation:
A primary source that could be used for a research paper on the US military's role in World War I is a letter written to a family member by an American soldier fighting in the war. This letter would provide firsthand accounts and personal perspectives on the experiences of the soldier during the war.
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