...[P]owerlessness in the face of economic free fall has emerged as a hallmark of the modern presidency. While Mr. Obama is facing a more acute economic crisis moment than his predecessors, characterized by a near depression, the truth is that every president going back to Jimmy Carter, at one point or another, has had to campaign or govern in an environment dominated by the same cyclical and stubborn factors — recession, unemployment, rising energy costs. And so perhaps Mr. Obama’s presidency, as it reaches its midway point, is best understood not in isolation, but rather as part of a longer and still undefined political moment....

Historians have different ways of looking at the question. But in interviews, several hit on the same basic theme, which is that Mr. Obama and his immediate predecessors have been forced to contend with the erosion of self-sufficiency....

The Stanford historian, David M. Kennedy, points out that for most of the 20th century, foreign trade accounted for roughly 10 percent of America’s G.D.P. That number started rising in the 1970s and now hovers at above 25 percent of G.D.P.

Mr. Kennedy said he suspected that his future fellow historians would classify our most recent presidencies as encapsulating an “era of globalization” in which “the whole concept of sovereignty is less meaningful than it once was.”

John Lewis Gaddis, the Yale historian and pre-eminent scholar of the cold war period, calls the last decade or so an “age of regression,” meaning that the popular notion of a “unipolar” world — one in which the United States was supposed to have no serious economic or military competitors — gave way to the realization that the best America could aspire to was a stable balance of power....

“This is what will end up defining this era of the presidency — the diminished power, the diminished authority, the diminished capacity to shape events,” says Robert Dallek, the presidential biographer. “It’s the presidency in eclipse.”...