America's greatness has greatly dwindled as a result of the judgment God has brought against her as a result of the nation's sin. What is occurring now will no doubt continue and grow worse unless God's people humble themselves and pray, and seek His face,and turns from this wickedness. And you make an excellent point. But notice, God doesn't place the impetus for healing upon the wicked changing, but on His own people to live as they already should be living.
That verse in particular does apply to God's people. How true that is. We should indeed take heed. It's the norm for Christians to marry unbelievers and repeat the sins of the OT.
It's been a while since I studied the topic, but there are 3 or 4 reasons that God judges nations. I'll only add one more for now.
There are many examples of God's judgement upon the nations in general.
Remember Nineveh, Sodom, Gomorrah and the other cities of the same plain?
There were certain sins that brought national or citywide wrath from on high.
He often called them Reprobate/ rejected.
Chapter 6 of Genesis, God started over with one family. Sadly, one of Noah's sons was a pervert. That led to the judgement of many heathen nations from his lineage.
Lastly, look at the world when Christ calls His people out.