In 1824, Henry Clay secretly finances an armed expedition against the new Confederacy. Led by Robert Crittenden, the attackers are a rabble of adventurers and freebooters, not settlers. They're out to make the blacks and Indians pay for taking what they consider theirs.
Driscol is expecting them. When they assault the Arkansas Post fort, his black soldiers kill them almost to the last man. It's a total rout.
Clay uses this defeat as a rallying cry. "Negroes massacre whites!" scream the headlines, without mentioning that the black soldiers were defending their country against an invasion he financed. Because of the fears he's stirred, Clay wins the presidency and begins planning a war of conquest.
Both Clay and Driscol expected and wanted Crittenden's defeat. Both expect and want a war between the US and the Confederacy. Both think they're going to win.
The difference is that Driscol understands the situation better. He knows his black and Indian followers are smart, tough, and dedicated to the new country's survival. Clay thinks they're a bunch of ignorant savages and children who will give up and surrender after a short war.
As I said, the Arkansas Confederacy seems to depend on Driscol's military and political acumen. Would it survive without him? Maybe, maybe not. Rather than an outright invasion, it might suffer the eventual fate of Oklahoma. Even though that was supposed to be an independent Indian territory, the US eventually forced it open for settlement.
The same thing could happen to the fictional Arkansas Confederacy in several ways. The US could play one tribe against another and coax some of them to sign away their rights. Or it could instigate a civil war between the factions and step in as the "peacekeeper." Or the US Supreme Court could rule that the treaty establishing the Confederacy was invalid.
US attitude = "live and let live"?
Reader Stephen disagrees with my assessment that the US wouldn't have tolerated another country in its midst:
There's no real conflict between the minority who wanted abolition and the majority who believed in America's Manifest Destiny. The key point is that nobody advocated a black-ruled country on the American continent. Even the abolitionists expected blacks to emigrate to Africa or become a permanent underclass in the US. Few thought they'd become full-fledged American citizens who literally had equal rights.
Therefore, I agree with DMarks when he said:
After all, the separate Mormon nation did not last and had to be taken over. And that wasn't even good farmland, and it was run by whites.
In fact, name one significant "native" or "foreign" culture that the US tolerated within its borders with a "live and let live" attitude. One significant culture that remained independent with no attempts to assimilate it. Good luck with your answer...you'll need it.
For more on the subject in general, see "What If" Stories About Indians and The Best Indian Books.
Below: America's "tolerance" for its un-American neighbors.