Is America still the engine of global economic growth?

In the first part of the decade, with Japan still stagnating and Europe flirting on-again, off-again with recession, analysts spoke over and over of America as the engine of global economic growth. This went on for so long that I simply ran out of train metaphors while writing for the Economist.

Prospects have gotten a little perkier over the last eighteen months; Japan seemed to be (finally!) pulling out of its decade-long doldrums, and France and Germany posted growth rates that were, if not exactly roaring back, at least very solid.

But the promise has not quite played out, as the Wall Street Journal chronicles:

The notion that the rest of the world has "decoupled" from the U.S. came into vogue earlier this year, as overseas economies -- particularly emerging markets -- continued to post robust growth and Europe and Japan appeared to be enjoying a long-delayed upturn.

Policy makers joined the decoupling parade. In the spring, the IMF included a chapter in its April World Economic Outlook called "Decoupling the Train." The gist: The current weakness of the U.S. economy stems largely from housing woes -- and housing is less global than, say, computers and other parts of the U.S. economy. That is good news for the rest of the world.

But the U.S. is now flirting with something more severe than a mere slowdown. That -- along with rising oil prices and the specter of a global credit crunch -- is changing the picture.

Europe is showing signs of faltering, while Japan may be at risk of sliding back into recession. While developing economies like China are still on a steady boil, recent drops in their stock markets suggest investors are beginning to doubt their immunity to a U.S.-led slowdown.

Germany and Japan may be growing, but they're extremely dependent on exports, which means they won't serve as the markets that fuel growth in the rest of the world. That, for too long, has been America's job. Now that we're ready for retirement, it seems we forgot to train our replacement.