The Last Slaves Freed in America

I am reading Alberty v. U.S. (1896):

Although the prisoner Alberty was not a native Indian, but a negro born in slavery, it was not disputed that he became a citizen of the Cherokee Nation under the ninth article of the treaty of 1866, 14 Stat. 799, 801, by which the Cherokee Nation agreed to abolish slavery, and further agreed "that all freedmen who have been liberated by voluntary act of their former owners or by law, as well as all free colored persons who were in the country at the commencement of the rebellion and are now residents therein or who may return within six months, and their descendants, shall have all the rights of native Cherokees."