Soon 20,000 U.S. soldiers will arrive in Europe. They will bring tanks, armored vehicles and other equipment. Even as some Europeans claim the trans-Atlantic relationship is unraveling, the U.S. is about to launch its largest military exercise in Europe in more than a quarter-century, known as Defender Europe 20. The first stages began earlier this month, and the exercise will continue through the spring. The next step should be a similar endeavor by the Europeans themselves.