The “ hirelings and slaves “ were Americans of all colors and socioeconomic classes who were being burned out of house and home and shelled by Britsh bombardments along the shores of the Chesapeake
American slaves faced a decision- join the British ( who ironically had trafficked most of them to the Americas) ); escape and migrate: or stand and fight for common values promised by America even if unevenly delivered and still in struggle .. The British sought to divide American resistance by offering sanctuary to slaves who would defect - but within limits since the British landholders in the West Indies relied on slavery
Here is one account of that struggle
http://www.pbs.org/wned/war-of-1812/essays/black-soldier-and-sailors-war/
The people of Baltimore - all of them - free men slaves rich poor tradesmen aristocrats - watching the battle of Ft McHenry from their windows were trapped, facing a common enemy as “ Americans”” and death and destruction of their homes regardless of loyalties ... Scott was pointing out that joining the British against America saved no one f rom its destruction
This is a good version of all 4 verses , dedicated to victims of 911
Who also died at the hands of a common enemy of American values
https://m.youtube.com/watch?v=piT33GV2EMA
No, they were the British.
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!
To put it in today's English,
What happened to the invaders who said they would come over here and take over our country? All dead or running away. Our flag still flies.