what does this quote mean - a nation that said all men were created equal, but kept my people in chain

Aminata is pointing out the irony of America's claim of being the land of the free and equality for all men yet kept black people in chains. Aminata harbors no love for the country in which she lives, neither Britain nor the United States. When she was made witness to the birth of the new nation, she was disgusted. She didn't want these people to succeed because she knew they didn't deserve it. Their success was made upon the trodden corpses of their slaves.