In the poem " Free Labor" by Frances Ellen Watkins Harper

Men! whose boast it is that ye

Come of fathers brave and free,

If there breathe on earth a slave,

Are ye truly free and brave?

If ye do not feel the chain,

When it works a brother's pain,

I think that "chain" relates to both slaves and people who are not slaves. Can anybody be truly free when another humanbeing is in chains. The metaphorical chains applies to everyone.