Why did Phule dedicate his Gulamgiri to the American movement to free slaves?

Phule dedicated his book ‘Gulamgiri’ to the American movement to free black slaves. As ten years before he wrote his book in 1873; the American Civil War had been fought leading to the end of slavery and racial discrimination in America. He mainly dedicated this book to those Americans who had fought to free slaves. He did this in order to establish a link in between the condition of lower caste in India and the black slaves in America.

In this book he also openly thanks Christian Missionaries for making the lower caste of India realize that they are worthy of all human rights too.