The plot revolves heavily upon promises made in person by President Abraham Lincoln to Chief Acoma just days before Lincoln's assassination. Lincoln never went to New Mexico (not even when it was a territory, as called in the film.)
After being informed that Acoma speaks English, Abe Lincoln says "of course he speaks English, he's been here longer than the white man", implying America was an English-speaking country before the Europeans arrived.
Hans Conried, who is uncredited in this, said of his role, "Every tall actor wants to play Lincoln."
There are a couple of comments about President Abraham Lincoln talking to Chief Acoma days before his assassination. The walking stick he presents to the Chief clearly has the date 1860 - Lincoln was assassinated in April, 1865, so the timeline between presentation of the gift and the assassination is not days, but is actually years. But it really doesn't matter, as ole Abe never made it to New Mexico as far as one knows.
The women on the stagecoach say they are on the way to Las Vegas. Las Vegas was founded in 1905 and incorporated in 1911. It didn't exist in 1865.