The third stanza goes as follows:
And where is that band who so vauntingly swore
That the havoc of war and the battle's confusion,
A home and a country, should leave us no more?
Their blood has washed out their foul footsteps' pollution.
No refuge could save the hireling and slave
From the terror of flight, or the gloom of the grave:
And the star-spangled banner in triumph doth wave,
O'er the land of the free and the home of the brave.
A listener of NPR's 1A said today that with "slaves", the song refered to black slaves that fought with the British to earn their freedom. However, online I found that it referred to the kidnapping of sailors to serve in their ships.
To whom is the word "slave" referring to?
This was written during the War of 1812. One of the many grievances the United States had with Britain was the impressment of Americans into the British Navy (and in some small part France and Spain too). It is possible that Key is referencing these people. However, Key was a slaveholder and could have also been referencing free regiments of former slaves the British Army recruited into the "Colonial Marines" and other units. Key never mentioned, nor was he ever asked, what he was referring to.
I'm going to argue that he's referring to the raised regiment of freed slaves. I base this argument on an analysis of his words.
None of that hints at sympathy with "impressed sailors" or else he would have said something along the lines of we freed our citizens or saved our suffering brothers. Furthermore, the battle took place between the 13th and 14th of September, 1814. The proclamation of recruitment that eventually resulted in the formation of the black Colonial Marines was issued in April of 1814 and encouraged slaves and settlers to seek out British forces and request enlistment or resettlement as free subjects of the crown. So the regiment that was raised was known to the Americans in and around Maryland by the time of the battle and penning of the Star Spangled Banner. Without any doubt. This was Key's backyard and the successful defense of Fort McHenry certainly assured that Key's own interests were protected and his slaves (or those of his friends) discouraged from fleeing. That is my best-educated guess.
Sources:
EDITED for formatting.