Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said the war is growing more complicated and the enemy gaining in sophistication. Winning will require more resources from outside Afghanistan, including more troops, Mullen told Congress.
"A properly resourced counterinsurgency probably means more forces, and without question, more time" and dedication, Mullen said.
Gen. Stanley McChrystal, the U.S. commander in charge of both American and NATO forces in Afghanistan, delivered a grim assessment of the war to Washington last month and is expected to follow up soon with a request for thousands of additional troops and more equipment.
That will leave Obama to decide whether to expand a war that polls say is rapidly losing public support in the U.S. and drawing pointed criticism in Congress. He has already roughly doubled the size of the American military force in Afghanistan since taking office, with only limited gains to show. Obama has an ambitious strategy to turn around a war that will soon enter its ninth year, and his aides say the plan needs time to work.
Mullen said he does not know how many additional troops McChrystal will request, but he left no doubt that the commander has concluded that the 21,000 U.S. troops Obama has already approved are not enough.
Sitting opposite Mullen, the Democratic chairman of the Senate Armed Service Committee was unswayed. Sen. Carl Levin of Michigan warned the White House last week that he does not want to see a request for more troops until the United States takes bolder action to expand Afghanistan's own armed forces.
"Providing the resources needed for the Afghan Army and Afghan police to become self-sufficient would demonstrate our commitment to the success of a mission that is in our national security interest, while avoiding the risks associated with a further increase in U.S. ground combat troops," Levin declared at Tuesday's hearing.
Several other Democrats have said they want a clearer timeline and measures of progress from the administration before approving large expansions of the troop commitment or mission. Congress has approved most of the money Obama requested for the war so far, but a large troop increase would probably require a separate add-on spending bill.
The head of the House's defense spending panel, Rep. John Murtha, D-Pa., this week questioned the logic of adding troops.
"In Vietnam it took 500,000 troops and that didn't solve the problem," the Vietnam veteran told the foreign policy blog The Cable. "We have to take a different approach."
Recent national polls indicate slipping support for the war and growing doubt that it can be won. The latest AP-GfK survey found that less than half - 46 percent - now approve of Obama's handling of Afghanistan, a 9 percentage point drop since July.
A CNN poll conducted this month said 39 percent supported the war and 58 percent opposed it. That compared with 53 percent supporting and 46 percent opposing in early April, days after Obama announced a new war strategy and vowed to provide resources in ways his predecessor had not.
Fifty-one U.S. troops died in Afghanistan in August, more than in any other month since the U.S.-led invasion in October 2001.
Mullen spoke at a hearing on his nomination for a second term as the nation's highest-ranking military officer. He is expected to win easy confirmation.
Mullen's remarks, cautious as they were, are the first clear marker in an internal debate over Obama's next steps. Defense Secretary Robert Gates has not made up his mind whether to support a troop increase beyond the current level of 68,000, his spokesman said. Gates has long worried publicly that too large a force in Afghanistan would be self-defeating because Afghans would see the troops as occupiers, but he has recently sounded resigned to at least a small expansion.
At the State Department, Secretary of State Hillary Rodham Clinton told reporters Tuesday that the administration would deliberate "for some time," suggesting no decision was imminent.
"Everyone is providing their best ideas and making their contributions about the way forward in Afghanistan," Clinton said.
The Senate committee's ranking Republican, John McCain of Arizona, said committing too few forces to the war would invite a rerun of mistakes the U.S. made in Iraq. "I've seen that movie before," said McCain, the 2008 GOP presidential nominee.
Although Mullen and other senior military leaders say the Afghan armed forces are the key to a successful U.S. exit from the war, Mullen suggested Tuesday that reliance on more Afghan training at the expense of full-on combat is a false choice.
"Sending more trainers more quickly will give us a jump-start, but only that," Mullen said. "Quality training takes time and patience."
Sen. Lindsey Graham, R-S.C., took the point further. When Mullen told him it would take two to three years to train enough Afghan troops to do the job, Graham asked what would happen in Afghanistan in the meantime.
"I think the security environment will continue to deteriorate," Mullen replied.
Then Graham made a larger point about public support in the U.S., one that hung over all the specifics of troop levels and trainers and the abilities of the Afghan government.
"Do you understand you've got one more shot back home?" Graham asked, mentioning the poll results. "Do you understand that?"
"Yes, sir. Yes, sir."