Updated

Defense Secretary Robert Gates held out hope Tuesday that U.S. forces might be able to start leaving Iraq before the end of the year, if daunting conditions including subdued violence and political reconciliation are met.

Gates told lawmakers the current buildup of forces by 21,500 troops is "not the last chance" to succeed in Iraq and conceded that he's considering what steps to take if it doesn't work.

"I would be irresponsible if I weren't thinking about what the alternatives might be," Gates told the Senate Armed Services Committee.

But he asserted, "We at this point are planning for success," and he described in sketchy form what could bring about the beginning of a withdrawal.

"It seems to me that if the plan to quiet Baghdad is successful and the Iraqis step up" by providing promised forces of their own and move toward resolving the country's bitter political disputes, Gates said, "I would hope that we would be able to begin drawing down our troops later this year."

Gates said last month that the troop increase seems likely to last months, not years. The outgoing top general in Iraq, George Casey, has said he hoped some of the extra troops could start returning home by late summer.

Gates was grilled on the war as the full Senate remained stalled on debating a resolution that would join most Democrats and some Republicans in a stinging critique of President Bush's course in Iraq.

His testimony came, too, with U.S. and Iraqi forces on the verge of opening their campaign to subdue the insurgency in Baghdad. Gates said the operation was to have started on Monday but "it's probably going to slip a few days, and it's probably going to be a rolling implementation."

Gates did not say what other options he was considering if the addition of U.S. forces fails to control the violence in Baghdad and western Anbar province, where the Sunni insurgency is based. But he and Gen. Peter Pace, chairman of the Joint Chiefs of Staff, sought to assure lawmakers that the additional troops pegged for Iraq will go there with sufficient equipment for the fight.

They said that a shortage of armored vehicles in one phase of operations will be remedied by July, and troops who need them will not be deployed from their compound in Iraq until the vehicles are supplied.

Pace said the number of ammunition-packed roadside bombs encountered by U.S. forces has doubled in a year, and these weapons now include a deadlier version coming from Iran.

Updated jamming equipment and other disabling tactics have proved effective in rendering more of the bombs harmless, he said, but U.S. casualties have remained about the same because of the higher numbers and deadlier nature of the explosives.

"The amount of ammunition available is incredible," Pace said. This, despite the clearing of 430,000 tons of ammunition from more than 15,000 sites.

Republican Sen. John Warner of Virginia pressed Gates on the question of what happens if Bush's plan doesn't work. "I have to believe we're thinking beyond the Baghdad operation," the senator said.

"It is not the last chance," Gates replied.

Bush's spending request marks the first time the president has offered an estimate of how much the wars in Iraq and Afghanistan will cost a year in advance.

Gates spoke a day after Bush sent Congress a proposed budget that includes nearly $142 billion for war costs for the next budget year starting Oct. 1. That would bring total spending on the Iraq and Afghanistan wars and the armed forces' worldwide efforts against terrorism to a massive $662 billion.

Gates acknowledged the "sticker shock" but said military spending is consuming less national wealth than during the Vietnam and Korean conflicts and after the Cold War.

A procedural vote Monday sidetracked debate on a nonbinding resolution stating Senate opposition to the buildup. Senate leaders sought to put the debate back on track.

"The president must hear from Congress, so he knows he stands in the wrong place — alone," said Senate Majority Leader Harry Reid, D-Nev.

Republicans denied they were trying to block a vote on the measure. They said they were seeking fair rules and consideration of a GOP alternative measure.

"As far as stalling, we've got a week here where we can have a full debate," said Senate Minority Whip Trent Lott, R-Miss. "But we insist on it being a full debate and a fair one."