Defense Secretary Donald Rumsfeld, facing sometimes hostile questioning Thursday from members of the Senate Armed Services Committee, defended his handling of U.S. forces in Iraq as two of his top aides testified that the country is on the verge of sectarian civil war.

"I believe that the sectarian violence is probably as bad as I have seen it," said Gen. John Abizaid, head of the U.S. Central Command, adding that if something is not done soon, civil war is inevitable.

Rumsfeld, however, while not agreeing with Abizaid's assessment, insisted that the insurgency in Iraq is "resilient," but can be defeated. He said it is up to the Iraqi leaders to find a way to reconcile internal divisions there. For now, however, the United States' commitment to Iraq fulfills its own security needs as much as Iraq's, he said.

Rumsfeld's steadfast defense of the administration's Iraq policy brought on a stinging attack from Sen. Hillary Rodham Clinton, who told Rumsfeld he was presiding over a "failed policy" in Iraq and Afghanistan.

"Under your leadership there have been numerous errors in judgment that have led us to where we are," the New York Democrat said. "We have a full-fledged insurgency and full-blown sectarian conflict in Iraq."

The showdown between Clinton, a potential presidential candidate in 2008, and Rumsfeld, the public face of the Bush administration's war effort, included the strongest criticism of the Iraq war she has made to date.

The defense secretary seemed briefly stunned by the intensity of her attack, exclaiming, "My goodness," before launching into a point-by-point defense.

He rejected some of her specific criticisms as simply wrong and said the war against terror will be a drawn-out process.

"Are there setbacks? Yes," said Rumsfeld. "Is this problem going to get solved in the near term? I think it's going to take some time."

Joint Chiefs of Staff Chairman Gen. Peter Pace supported Rumsfeld's assessment, and said the ultimate success of an Iraq transition to democracy fell with the country's leadership and people.

"Shiite and Sunni are going to have to love their children more than they hate each other," Pace said. "The weight of that must be on the Iraqi people and the Iraqi government."

Rumsfeld also vigorously defended the performace of American troops, who have come under fire recently for alleged atrocities against Iraqi civilians.

"Americans didn't cross oceans and settle a wilderness and build history's greatest democracy only to run away from a bunch of murderers and extremists who try to kill everyone that they cannot convert and tear down what they could never build," Rumsfeld said.

"We can persevere in Iraq or we can withdraw prematurely until they force us to make a stand nearer home. But make no mistake, [extremists] are not going to give up whether we acquiesce to their immediate demands or not," he said, adding, "Ultimately the sectarian violence is going to be dealt with by Iraqis."

All three witnesses said the problem will endure for some time to come, but for now, the United States must offer the training for Iraqi forces and military prowess to degrade those irregular fighters.

Abizaid said the top priority in the Iraq war is to secure the capital, where factional violence has surged in recent weeks despite efforts by the new Iraqi government to stop the fighting.

Sen. John McCain, R-Ariz., in questioning why the U.S. military was adding troops in Baghdad when the violence is being spawned by an internal conflict, argued that a buildup is likely to cause more U.S. casualties.

"The Iraqi forces need to benefit from our command and control abilities," Abizaid said, adding that the police units in Iraq did not develop the way the U.S. thought they would or should, and that the infestation of militia members loyal to sectarian leaders is troubling.

"I think there is a demand for military, political and diplomatic activities that move Iraq toward stability ... and I believe this current government, which is a four-year government, has that opportunity," he said.

Iraq's president, Jalal Talabani, said Thursday in Baghdad that his government is "highly optimistic that we will terminate terrorism this year. The Iraqi forces will take over security in all Iraqi provinces by the end of this year gradually, and if God's will, we will take the lead." After the comments, his staff sought to explain that Talabani was referring to the beginning of a "process" for Iraqis to assume control, not the final step.

Rumsfeld said that the conflict in the Middle East demonstrates that "what we're seeing is the face of the 21st century. These wars that we're engaged [in] and we see are not between militaries only. They are a clash between systems, political, economic and military, and they are fought with asymmetric warfare ... which is very much to the advantage of the attackers," he said.

None of the challenges, however, means the United States should bail from its commitments in Iraq, Afghanistan and elsewhere, Rumsfeld said.

"Iraq is only one part of a broader regional struggle underway. ... The vast majority of the people in the region do not want extremists to win," Abizaid said.

Pace said he knows that terrorists are wrong to think the United States has grown weary because re-enlistment rates are "at record numbers" and the American people do not want their way of life interrupted.

"We are a nation at war, fortunately most of our fellow citizens are not affected by that war every day. Our enemy knows they can not defeat us in battle, they do believe however they can wear down our will as a nation. They are wrong," Pace said. Winning the global War on Terror "will not be easy, this will not be quick and this will not be without sacrifice, but we will persist and we will prevail."

The testimony before Congress, already bitterly divided over the war, was not helped by fresh reports that say up to two-thirds of the Army's combat units are unprepared for wartime missions because of the strain of operations in Iraq.

The Pentagon this week announced its decision to extend the tours of an Alaskan Army brigade to bolster security around a volatile Baghdad and push troop levels to roughly 135,000 — dashing the Bush administration's hopes of dropping the figure by tens of thousands by the fall congressional campaigns.

Rumsfeld has said that he, of course, is conscious of the pressure to bring troops home, but it's not something that can be decided by domestic political debate.

"It's an art, not a science" to determine the right number of American forces there and when they should come home, he said. "Decisions about conditions for a draw down of our forces in Iraq are best based on the recommendations of the commanders in the field and the recommendations of the gentleman sitting beside me."

Democrats in Washington have highlighted the Army readiness issue as an example of the administration's mishandling of the war. They urged the president this week to begin pulling troops out of Iraq by the end of the year. Sen. Jack Reed, D-R.I., also got into heated discussion with Rumsfeld about the ability of the military to fix equipment damaged in Iraq. Pace said budgetary limits have an impact on that.

The military does "not have enough funding currently to provide for and repair all the equipment that currently sits at our depots waiting to be repaired," he said. McCain warned that any future budget requests relating to the war planning in Iraq should be part of the annual budget and not made in the form of emergency supplementals.

Bush consistently has said no pullout will begin until the fledgling Iraqi government can secure its position and Iraq's security forces can defend the country. Republicans have backed their GOP president on the issue, but have acknowledged their frustration with the length of the war and the delayed homecomings.

"That's a very difficult thing for us," said Sen. Ted Stevens, R-Alaska, of the Pentagon's decision to keep in Iraq some 3,500 members of the 172nd Stryker Brigade Combat Team, based at Fort Wainwright in Alaska.

Rumsfeld's appearance before the committee was decided at the last minute. He and Secretary of State Condoleezza Rice were scheduled to meet privately with senators on Thursday, and Rumsfeld had said Wednesday he was too busy to appear before the Senate panel.

He changed his mind after pressure from Senate Democrats, including Clinton, who said the Pentagon chief should be accountable to the public by answering questions on the war.

Rumsfeld did not say why he changed his mind, but the Pentagon denied that he was ordered by the president to show up. Officials note that the defense secretary meets with lawmakers fairly regularly, and testified at an appropriations hearing earlier this year and at other classified briefings.

Rumsfeld's relations with Congress have been testy at times and he occasionally has resisted testifying publicly on contentious subjects, including the debate over whether high-level officials should be held accountable for the Abu Ghraib prisoner abuse scandal.

FOX News' Molly Hooper and The Associated Press contributed to this report.