Final answer:
A document about the War of 1812 would be most useful for studying colonial reactions to conflict with the British government, the surge of national pride, and the subsequent westward expansion.
Step-by-step explanation:
A historian studying the War of 1812 and its impact on American nationalism would find a document about this conflict particularly useful. Such a document might capture the causes of war, such as British impressment of American sailors and support for Native American opposition to U.S. expansion, the conflicts during the event, and the aftermath, including its consequences on national pride and westward expansion.
The document would be most useful for studying colonial reactions to conflict with the British government and understanding how the War of 1812 fostered a surge of national pride and solidified the United States' sense of identity separate from Britain. It could also be insightful in analyzing how this pride contributed to the eventual expansion in the West.