The Founders were dealing with principles.
They knew history and saw the end of all great Empires was the same-destruction.
Now, we have outlasted the Soviet Union with its vast military might and we will defeat the forces of Islam as well, if we do not destroy ourselves first.
The US need not fear any single or group of nations.
We are vulnerable to acts of terrorism as is any nation, but the way to protect us from that is to get control our borders and immigration policy, which works against our defense needs.
If the United States falls, it will not be because a foreign enemy destroyed us, but because we destroyed ourselves with our own bad policies.
We are not greater or smarter then the great empires of the past that also destroyed themselves by trying to control more then they were able to.