This is not unique to the US and not even unique to the modern era. The "common enemy" is the glue of every civilization. Build a case against a scapegoat and rally public opinion to wage a war - this is essentially the job of every form of government. Let's wage war on poverty, corporations, the 1%, unions, drugs, immigrants, corruption, terrorism... the list goes on and on. You can go back to the Bhagwad Gita, the Bible, the ancient myths... and the story doesn't change very much.