“Right now, Iraq, along with getting Osama bin Laden, succeeding in Libya, and restoring the U.S. reputation in the world, is a clear plus for Obama,” said David Rothkopf, a former official in the administration of Bill Clinton and a national security expert. “He kept his promise and got out. But the story could turn on him very rapidly.”

For instance, Mr. Rothkopf and other national security experts said, Prime Minister Nuri Kamal al-Maliki of Iraq is swiftly adopting policies that are setting off deep divisions among Sunnis, Kurds and Shiites. If Iraq fragments, if Iran starts to assert more visible influence or if a civil war breaks out, “the president could be blamed,” Mr. Rothkopf said. “He would be remembered not for leaving Iraq but for how he left it.”…

Even if the unrest rose to levels approaching civil war, American officials said, it was unlikely that Mr. Obama would allow the American military to return.

“There is a strong sense that we need to let events in Iraq play out,” said one senior administration official. “There is not a great deal of appetite for re-engagement. We are not going to reinvade Iraq.”