Why did God do this? So that George Washington could become President of the United States, of course. At least, so says the pagan quoted in Under God:
Listen! The Great Spirit protects that man [pointing at Washington], and guides his destinies--he will become the chief of nations, and a people yet unborn will hail him as the founder of a mighty empire. (13)Did God protect George Washington through first the French and Indian and then the Revolutionary Wars? Of course.
Did God do this so that Washington could become President? Apparently so, since that's what happened.
But should we stop there? To be fair, the authors of Under God do not stop there, they go on to point out that Washington was very grateful to God (though Washington says "Providence" in the citation provided) for his preservation, and that this whole story (including Washington's gratitude) used to be found in American history textbooks.
So what happens if we ask a slightly different set of questions:
Did God protect Osama bin Laden through the war with the USSR in Afghanistan? Of course.
Did God do this so that Osama bin Laden could become the leader of Al Quaeda? Apparently so, since that's what happened.
And I suspect we can assume that bin Laden was likewise quite grateful to God for his preservation, though I suspect that particular side of the story will not be found in American history textbooks for, well, ever.
Hopefully you can see how this view of history quickly becomes very problematic. If we don't have some other way of deciding what matters, we have trouble seeing the difference between George Washington and Osama bin Laden. Fortunately, as Christians we do have another way of deciding what matters.
Let me explain by way of a big-picture view of history. In the Christian perspective, the teleology of history (goal towards which all human events are moving) is the glorification of God in the salvation of His church. This means that every human event that happens in history is being directed by God towards this end. And while we do not necessarily understand how these events exactly bring about that end, we know at least that none of them are ends in themselves. God did not preserve George Washington through the French and Indian War only so that he could be president, He preserved Washington ultimately so that the Gospel would reach those who have been set aside for salvation.
The problem is that we lack the perspective to say more than this on this side of heaven. Because of this lack of perspective, our temptation is to commit idolatry by pointing out what we can see (say the creation of a nation, be it ours or someone else's) and declare that event, person, or action to be God's teleological end. (Some people go so far as to identify this end of history with themselves, but that's perhaps a blog post for another day.) As Christians we must resist this temptation, even when we see others go down that road. We should instead simply confess that we do not understand how every event in history moves towards the salvation of the church, but that we believe it does despite our ignorance.
Someday we will glorify God for his hand at work in history, but it will not be finally for the creation of America. We will praise and glorify God for his creation of a spotless and pure bride from a rebellious whore by means of the blood of Christ. The historical steps to that end are important and should be a source of great delight and interest to Christians, but we should also remember that they are mere steps, and that it's not our place to decide that one step is more important or valuable than another. Such things are best left to God.