The report said the surge provided no breathing space or resolved any political issues, Middle East Newsline reported. Shi'ites, Sunnis and Kurds continued to fight over Iraq's oil resources, particularly in Kirkuk.

"All of these questions have led to violence in the past, and all of them almost certainly are going to lead to violence again," the report said.

The report said the U.S. military marked the glue that has held Iraq together. The institute warned that Iraq would face increasing internal threats amid any U.S. military withdrawal.

"The more troops we withdraw, the more we start withdrawing them from the riskier areas that are less secure or where Iraqi forces are unreliable," the report said. "There are a lot of little Saddams in the Iraqi military. The fewer American eyes on them, the more that Saddamishness will come out."

The report envisioned Iraq engulfed by a civil war in wake of a U.S. military withdrawal. Ricks said such a war could spill from Iraq and spread throughout the Gulf region.

Given such a scenario, the report expected President Barack Obama to maintain up to 50,000 troops in Iraq indefinitely. Ricks said the U.S. president has come to realize "just how bad the trouble he is in."

"He's eventually going to have to settle into a long war with much smaller numbers of forces — 35,000 to 50,000 troops — but probably for several more years of fighting," the report said.

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