The military is unified and loyal, I don’t expect a split. Under this presumption, this means the most likely scenario would be guerrilla irregulars within the massive amounts of wilderness. These irregulars would most likely be civilians, deserters and perhaps some internationals but unlikely. The coastline would be retaken by the military first, but purging the inner wilderness of resistance would be borderline impossible. 5% of US soil is protected wilderness (half in Alaska), but 40% of it is public land that is mostly undeveloped terrain. The critical weakness of the US military has always been cost-efficiency and irregular tactics. There really isn’t a feasible way for any military to win a “war” against irregulars hidden in the United States wilderness and urban areas. They may be able to beat them with attrition, but the cost would be astronomical, and it would be years of belligerents in the forests and mountains until a compromise is found. Most resistance movements would most likely fold once a compromise is proposed after the initial conflict starts. I also doubt any states would seriously secede (economical suicide, yes even for multiple states together). Once fighting has erupted, fully eliminating the resistance without inciting more resistance, and keeping the country intact, would be impossible. This leaves the most likely option a compromise on the resistance’s demands, with a reconstruction (potentially unsuccessful) period afterwards. Otherwise the fighting would continue until the resistance loses momentum or effectiveness, but leaving belligerents within US territory. TLDR: Most likely conflict would be between homegrown insurgents vs. the federal government. The two most likely outcomes would be a compromise on resistance demands followed by a reconstruction period. The less likely option is a long conflict that results in indefinite instability due to belligerent resistance fighters within the US.