But I doubt that letting Russia take a small NATO country would cause the collapse of U.S. power abroad. Paradoxically, it might increase our power

In Eastern Europe, the pro-Russian people would be like: "See? The West is toothless; Russia will regain her former sphere of influence soon (which includes us)." And people have an instinct to side with the winner, so the people who don't have strong political opinions would be likely to join what seems like a winning side.

The map drawn at Yalta Conference was a Schelling point for decades. People still remember it.