This is an illustration from a book about the Old West. The actual document hangs in the Fort Smith History Museum in Fort Smith, Arkansas. The caption just above the document is incorrect.
Your mission is to determine why it is incorrect. The document contains many pieces of evidence that will eventually lead you to the solution. At the bottom of this page you will find a number of hints that can be followed to the eventual end. If you need to use these hints (and you will), do so in the order that they are presented. Each hint has a separate solution which can be accessed by the link at the bottom of the hint page.
We will start you off (and save you several hours) by telling you that none of the preprinted material above the handwriting contains any clues. However, the fact that this takes place in Fort Smith is of some importance.