During the War of 1812, the British were also at war with France, and they needed more troops to fight the French. In order to keep troops in the American front, the British usually recruited Native American allies, mercenaries and also Black slaves in exchange for freedom after the war. Given the conditions they were facing, it is no wonder that Native Americans and African slaves would fight for those promising freedom. Still, many Americans like Key himself, saw this as a dangerous maneuvre, encourging a racial war and unleashing "savages" against Christians.
On the other hand, as a prominent attorney, he was sent to negotiate the release of American prisoners with General Robert Ross and Rear Admiral George Cockburn. The negotiations with these naval officers were succesful, and they are not mentioned in any manner in the poem, so Key probably did not feel that animosity against them. Still, the fact that British officers were the ones enlisting these "hirelings and slaves" would probably reveal some rejection.
