BERLIN — Remember America’s pivot to Asia? Just over three years ago, President Obama announced that Washington would rebalance its resources to a part of the world where, it was believed, the decisive battles of the 21st century would be fought.

It seemed to make sense. China was the big story. The United States was tired of the Middle East. And Europe, finally pacified after the Cold War and the Balkan conflicts, didn’t seem to pose much of a security challenge.

Today, though, the pivot to Asia appears to have been largely called off. The Middle East, with its revolutions and power vacuums, is sucking America back in. Meanwhile, the European Union’s failure to stop the Russian invasion of Crimea means that the United States has again been drawn into continental politics, with the looming possibility of another Cold War. And both Europe and the United States are now making a quietly revolutionary attempt to rewrite the rules of trans-Atlantic trade.

This is a remarkable comeback for America. In Europe’s last crisis, over the euro, the United States was largely absent — partly for a lack of financial resources, and partly, it seems, out of frustration with the muddled way that the union went about saving itself.