The Dawes Act provisions continued for almost 50 years, resulting in the fractionation of the allotments to the point of ridiculousness—hundreds, even thousands, of heirs of the original allottees co-owning tiny tracts of land and earning next to nothing on what that land was worth.

FDR's Commissioner of Indian Affairs, John Collier, decided that it was high time to stop trying to destroy Native American culture by encouraging them to assimilate into American society. He saw the financial and cultural devastation this program had visited on the tribes and wanted to give back sovereignty to manage their lands and reconnect with their cultural traditions.

The Wheeler-Howard Act stopped all future allotments, and prohibited any individual holding one to sell to anyone except a tribe member. It gave un-allotted tribal land back to the reservation of the tribe that was living there. And it made that land tax-exempt.

Oklahoma was a sad exception. It's not that Collier had anything against Oklahoma—in fact, he was a huge Hugh Jackman fan—it's just that there was no reservation left.

The act gave the Secretary of the Interior a budget to use to help with economic projects on the reservations, and allowed him to set up new reservations if requested by a recognized tribe. Tribal councils would be allowed to write their own constitutions, and neither Congress or any other government agency could make policies that would affect the privileges of the tribes (source).

It was a first step towards the U.S. government recognizing that Native American culture was worth preserving, and that the Native American nations could act as sovereign nations with minimal interference from the feds.