WASHINGTON — Lieutenant General David Petraeus said he couldn’t successfully carry out his new mission of leading American forces in Iraq and providing security for Baghdad without the additional 21,500 troops that President Bush plans to send.

During a confirmation hearing yesterday before the Senate Armed Services Committee, General Petraeus said it would discourage American forces and encourage their adversaries if Congress passed a resolution expressing disapproval of the increase in American forces.

“This is a test of wills at the end of the day,” General Petraeus, 54, said. “A commander in such an endeavor would obviously like the enemy to feel that there is no hope.”

General Petraeus, who holds an international-affairs doctorate from Princeton University, offered a picture of what might happen if America left Iraq before the violence has been quelled. He said sectarian groups would “stake out their turf” through ethnic cleansing, neighboring countries might intervene on behalf of their Iraqi allies, terrorists might lay claim to part of Iraq, and oil production might be disrupted, harming the global economy.

The Army general offered no guarantee of success even with the added troops, while reassuring skeptical lawmakers that a positive outcome was still possible.

“The situation in Iraq is dire,” he said. “The stakes are high. There are no easy choices. The way ahead will be very hard. But hard is not hopeless.”

General Petraeus said the increase in troop strength was tied to a shift in mission for all American forces in Baghdad. He said their focus would now be to protect the capital’s residents from violence perpetrated by insurgents and sectarian militias, rather than to prepare Iraqi forces for a handover of security responsibilities.

“The security of the population, particularly in Baghdad, will be the focus,” he said. “To carry out the new strategy, the additional forces directed to move to Iraq will be essential.”

Those comments represented at least an implicit criticism of the outgoing American commanders in the theater, Army Generals John Abizaid and George Casey, who pursued a strategy centered on training Iraqis — although Petraeus made a point of saying that Generals Abizaid and Casey supported the new plan.

On the other hand, General Petraeus’s statement about the impact of a congressional resolution disapproving the Bush strategy was taken as a rebuke by committee Democrats who favor such a move. His comment on the subject, given in response to a question from Senator McCain, a Republican of Arizona who strongly supports adding troops to Iraq, touched off a rhetorical clash among lawmakers.

Senator Clinton, a Democrat of New York, retorted that there was “good reason” to be skeptical of Bush’s new strategy.

“What those of us who are issuing resolutions and statements of disapproval fear is that you are being sent to administer a policy that, frankly, does not reflect your experience or advice,” Mrs. Clinton told General Petraeus.

“You wrote the book, general, but the policy is not by the book,” she said. “And you are being asked to square the circle, to find a military solution to a political crisis.”