We are also told that He will bless whom He will, and curse whom He will. He may bless a nation with which He is not pleased (such as Babylon) so that nation will be used as a weapon against a nation which He wishes to discipline (such as Israel). The situation is far too complex to make up magic formulae over (i.e. "We will continue to be strong as long as we bless Israel.").
IMHO: If G-d were going to judge anybody for the U.S. descent into the pit of moral relativism (read "immorality") it would be the Church, every single last denomination, not the government. With some magnificent exceptions, the Church has consistently failed to:
If you want G-d to save America (with apologies to FReeper John O.), pray that He will restore the Church. Shalom.
However, it is nonetheless true that God views some nations more positively than others, and that He causes nations to rise and fall.
Our condition is the church's fault:
2 Chronicles 7:14 If my people, which are called by my name, shall humble themselves, and pray, and seek my face, and turn from their wicked ways; then will I hear from heaven, and will forgive their sin, and will heal their land.
Note that He says "MY PEOPLE". We, as Christians, have the responsibility to pray and seek his face for our land. We have not consistently done so.
But in some of our churches that is changing. We are now actively seeking God's blessing for our country in at least the church I attend.