Eurasia Group today published its annual list of the main geopolitical threats for 2021. For the second year in a row, the #1 Top Risk is rising political polarization in the United States, which not many years ago was deemed one of the world's most stable nations, with strong institutions and — as the sole global superpower — with a clear mandate to lead the world on many fronts.

That's all gone, for now. Why, and what does this mean for America and other countries?