Bootnote: The Land of the Free - Ha!
Our standard note on why this phrase is a bit silly
*Slavery was shown to be untenable under English law (though not in other parts of the British empire) by the Somerset v Stewart case in 1772, some years before American independence. The triangular slave trade in which African slaves were shipped mostly to the Americas was outlawed by Britain in 1807, and thereafter the primary effective force against the slave trade was the Royal Navy.
One of the strongest protections a slave trader could have from then on was the flag of the United States. In the "Land of the Free", slavery remained a legal institution in many states, and a slave was still a slave even if he or she could reach a "free" state - the only way to achieve freedom was to reach British territory. This only changed with the Civil War, and slavery in the USA was formally abolished in the Thirteenth Amendment of 1865.
Whatever US independence may have achieved, it wasn't a good thing for freedom. ®