Balkanized North America is one of the more common AH cliches.
Basically, what it comes down to is North America is divided into several more countries, particularly in the area currently occupied by the United States, Canada, and Mexico. Common PODs include the lack of cooperation at the Constitutional Convention of 1787 and a more disastrous Civil War for the Union in the 19th Century.
In several scenarios, the northern United States and the southern states are their own countries, as well as New England, Utah (Mormon Deseret), and the West Coast (California). Texas is also usually independent.
Usually these scenarios are developed because the author wants to recreate Europe in North America, or because he wants to fully explore the budding nationalisms that several regions in the USA had throughout the years. They're usually good-reading, but also usually require a suspension of realism on the side of the reader in at least some instances.