Certainly, there are Biblical examples of nations that were warned of their imminent destruction by God through the prophets, and of nations that repented and avoided that destruction. Ninevah being warned by Jonah is an example. He didn't even tell them to repent, but only told them of God's coming wrath. They took it upon themselves to repent and seek God's forgiveness which He granted.
The United States is a different story. The Bible prophecies are already written and will happen. If the U.S., currently the most powerful and imposing nation on Earth, is not in those prophecies, then our fate is sealed. Repentance now will save individuals, but not our nation.
As Christians and patriots we have to ask ourselves if we really want America to continue in her current form? Our direction as a nation seems to be a headlong flight from righteousness into utter corruption, the kind of corruption (child murder, homosexuality, lawlessness, etc.) for which God destroyed many ancient nations.
Does anyone honestly see a turnaround on the horizon considering the moral decline of our nation? Or are we heading into the logical outcome of a depraved hatred of God that would explain America's absence from Bible prophecy?
I'm not suggesting that we surrender, but regardless of America's future, we need to remember:
1. who will ultimately win (God Almighty)
2. whose side we should be on (His)
3. that we need to be about our Father's business (spreading His Gospel, living in righteousness, helping the helpless)
4. that our best fighting is always done from our knees.
Please recall that there are those who have attributed the Daniel / Revelation prophecies to the Roman Empire; Communist China (Christians under the "Great Leap Forward"); and even Mussolini.
It just might be that our Nation is restored; and the prophecies are not about "us" anyway.
Cheers!