Complex sentence: Native Americans were not automatically considered U.S. citizens at that time, although by 1924, when a constitutional amendment granted them citizenship and voting rights, two-thirds had already gained the franchise through moves off reservations and other actions.
Split into simpler sentences: Native Americans were not automatically considered U.S. citizens at that time. In 1924, when legislation was passed granting them citizenship, voting rights were not ensured, but two-thirds had already gained the franchise through moves off reservations and other actions.