THE AMERICAN ECONOMY seems bedeviled by twin towers. The terrorist destruction of the World Trade Center has left a residue of homeland security costs that will be a drag on the economy for years to come. Then there are those financial twin towers–the trade and budget deficits–that have caused a periodic outbreak of nervousness among policymakers for decades. Now we have a new pair of threats, the towering pile of U.S. government IOUs held by China, and the increasing number of barrels of imported oil that America needs to keep its factories operating, its trucks rolling, and its citizens moving from meeting to meeting. Last week the vulnerability of America’s economic recovery to these threats became obvious.