The top three single nations in the world today are the United States, China, and Russia. At least two of these are presumably overseen by territorial, demonic princes (Dan. 10:13, 20).
Probably the governments of all three nations are overseen by Dark Spirits. Satan did claim, to the Lord Jesus, that all the kingdoms of the world belonged to him. Jesus didn't dispute Satan's claim, not that that necessarily validates it.
In any case, Satan only claims authority over the governments of the world (basileia is the word used in both Matthew and Luke), NOT the peoples (ethnos) of the world.
America began as a Christian nation not because of its government, but because of its people. Paul Johnson, in his book, "The American People" points out that in the early days of the nation, Americans read the Bible, studied the Bible, discussed the Bible, amd lived the Bible. The Founders could have all been pseudo-Christian deists, and America still be a Christian nation, because Americans were Christians.