A description of the world, and all the old postings, are at http://www.alternatehistory.com/decadesofdarkness/ - new postings, of course are available at the feed.
The general idea is that President Thomas Jefferson died earlier than he did in real life... while he was still in office in 1809, to be precise. Because of that, the states that sent delegates to the Hartford Convention (which happened sooner [December 1809 - January 1810] in this time line) ended up seceeding from the United States and becoming the Republic of New England, causing the War of 1811 (Second War of Independence), and the history that happens after that.