In 2030, a major earthquake sinks most of Southern California into the ocean. With the United States government focusing on the disaster on the ex-West Coast, the needs of the other parts of the country go unmet. In 2031, Utah announces its intention to secede from the United States, declaring its independence and annexing what is left of Arizona and Nevada. Inspired by Utah's example, fringe groups in Texas declare independence as well.
Also inspired by them are the Northwest Secessionist Forces, made up of various paramilitary groups in Wyoming, Montana, Idaho, Washington, and Oregon, who band together with Utah and Texas. In response, the US government intervene.
While the details of the conflict are unknown, these efforts at secession all fail and the would-be secessionists are pushed back to the margins of society. However, martial law still exists in some portions of these states.
One of the legacies of the Northwest War are FBI-run "good behavior" camps.
Despite the country being back together, the NSF continues to be a problem. Comments by JC Denton hint that the US might be on the verge of a third civil war.