God, in His sovereignty, has set in motion the principle of reaping and sowing. America is sowing bad, it is reaping bad. America is under the same judgment as Egypt, whose citizens breathe out murderous threats against Israel every day, along with a host of other nations. Many nations in Africa starve their own people, they are brutal in every way, they are under judgment as well.
Can the US repent and forestall the day of the Lord? I don't see any indication in scripture this is possible. Yes, even including the example of Nineveh, which is not a valid example for us today, we are under a different dispensation. Look in the new testament and find what God says of the nations, they stand condemned already. You're welcome to find a new testament example for your case, I'm listening. Gods Kingdom, which will supplant all kingdoms, could have came 2000 years ago, but God had a different plan. Someday real soon, the plan will kick back in, God will drag all nations into judgement. Its that simple.