The Army chief of staff, Gen. Peter J. Schoomaker, cautioned that people not read too much into the planning, because it is easier to pull back forces than to get units prepared and deployed at the last minute.
"This is not a prediction that things are going poorly or better," Schoomaker told reporters. "It's just that I have to have enough ammo in the magazine that I can continue to shoot as long as they want us to shoot."
His comments come less than four weeks before congressional elections, in which the unpopular war in Iraq and President Bush's policies there are a major campaign issue.
Mr. Bush acknowledged in a that "these are tough times" in Iraq, where sectarian violence has surged recently. But, he added, "it is in our interests that Iraq succeed."
President Bush defended his Iraq policy, saying that he had adjusted tactics to reflect changing conditions on the ground.
He was asked about a recent comment by the Republican chairman of the Senate Armed Services Committee, Sen. John Warner, that Iraq was drifting "sideways" and that the U.S. should consider major changes if the Iraqi government doesn't get the violence under control within the next few months.
"I appreciate Sen. Warner from going over there and taking a look," said Bush. "I completely agree."
Still, he insisted, "We're constantly changing tactics."
Last month, the top U.S. commander in the Middle East, Gen. John Abizaid, said the military would likely maintain or possibly even increase the current force levels through next spring. There are 141,000 troops in Iraq, including about 120,000 Army soldiers.
In recent months the Army has shown signs of strain, as Pentagon officials have had to extend the Iraq deployments of two brigades in order to bolster security in Baghdad and allow units heading into the country to have at least one year at home before redeploying.
Meanwhile, a controversial new Iraqis have died due to the war, suggesting a far higher death toll than other estimates.
Within hours, the president dismissed the study.
Mr. Bush told reporters at a White House news conference: "I don't consider it a credible report."
"Neither does General Casey, neither do Iraqi officials," President Bush added.