September 21 in Military History — On This Day

⚔️📍 West Point, New York1780

Benedict Arnold's Treason Revealed — American Revolution

British Major John André was captured with papers revealing that American General Benedict Arnold had agreed to surrender West Point to the British for £20,000. Arnold escaped to the British lines; André was hanged as a spy.

Benedict Arnold's betrayal — his name became the American synonym for traitor — nearly gave the British the strategic fortress at West Point that controlled the Hudson River and could have cut New England from the rest of the colonies.

Outcome

Arnold escapes; André hanged; West Point saved

Battle of Monterrey

A fierce four-day urban battle for the heavily fortified city of Monterrey in northeastern Mexico. Taylor divided his force, sending Worth's division to cut the western road while he attacked from the east. American soldiers fought house-to-house through the city's narrow streets, using picks and crowbars to break through walls rather than expose themselves in the open. After heavy casualties on both sides, Ampudia negotiated an eight-week armistice and withdrew his army — a decision that angered President Polk.

Monterrey was the most contested battle of Taylor's campaign and revealed the high cost of urban warfare. The armistice controversy exposed growing tension between Taylor and the Polk administration. The city's capture gave the U.S. control of northeastern Mexico and elevated Taylor to national hero status, launching his political career.

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