Written by the lawyer Francis Key (owned servants and represented slaves in freedom cases) after what inspired him at Ft. McHenry, that stanza refers to...

Hireling: A British mercenary
Slave: The British navy practice of excepting former American slaves and forcing them to serve for their freedom. It was a way to hit at the American economy and gain fighting men.

On the American side, it was the free blacks at the Battle of Baltimore that was the cornerstone of a defensive force (1,000 men) that decimated the British Navy (4,500+) which at the time was the worlds most robust! Blacks also fought in militias at Lexington and Concord, Yorktown, and Boston to name a few. Eventually, they were even enlisted in the Continental Army.

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