Here's a couple of starters:

1830-45 Choctaw Chief Nitakechi/Ambrose Williams
http://www.genealogy.com/forum/regional/states/topics/ok/28520/

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John T. Williams, Choctaw
https://www.accessgenealogy.com/native/john-t-williams-choctaw.htm

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There are quite a few Union soldiers by this name, here's one who was killed during the war

Ambrose Williams

Residence Benton IL;
Enlisted on 2/16/1864 as a Private.
On 2/20/1864 he mustered into "K" Co. IL 13th Cavalry
He was Killed on 7/10/1864 at Pine Bluff, AR

Sources used by Historical Data Systems, Inc.:
- Illinois: Roster of Officers and Enlisted Men
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