The US isn’t a democracy though, it is a dictatorship of the bourgeoisie, where every election we get to select which group of oppressors will wield state power against us until the next one.
Also decline is caused by anticolonial resistance and the logic of capitalism breaking down. I would suggest reading Lenin’s “imperialism” and Fanon’s “wretched of the earth”
https://i0.wp.com/geopoliticaleconomy.com/wp-content/uploads/2022/04/Ukraine-borders-historical-map-expansion.jpg?resize=880%2C592&ssl=1