I would recommend reading up on American history a bit more. The US has always been a regionalist country, with a weak federal government. As for the decline of the US empire — not sure what you mean here, but if you mean the waning of American influence abroad, that’s already happening and it’s voluntary. If you mean the decline of the country internally, such as reduction in quality of life and decline in GDP/output, this is already happening with the demise of globalization, but the US will be the least impacted, not most, owing to strong demographics, variation in climate, rich natural resources, and ability to integrate immigrants.