Whether or not they recognize it officially in a pledge of allegiance or some other way, the fact is that all nations are "under God." They were formed by God in the first place, are being evaluated by Him, and eventually have been or will be judged by Him.

“And they shall be mine, saith the LORD of hosts, in that day when I make up my jewels; and I will spare them, as a man spareth his own son that serveth him” (Malachi 3:17).

“Have not I commanded thee? Be strong and of a good courage; be not afraid, neither be thou dismayed: for the LORD thy God is with thee whithersoever thou goest.

“And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book” (Revelation 22:19).

“For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book” (Revelation 22:18).

This sober warning almost at the end of the Bible was given by the glorified Lord Jesus Himself (note v. 20), so should be taken very seriously.