He also revealed that the Pentagon could send two more brigades there by late spring and a third by mid-summer in an effort to try to salvage a country besieged by corruption and increasing violence.
But Gates said there can be no mistaking the realities of the situation on the ground in Afghanistan. He said the United States must ratchet down its expectations in the war - and that the new aim should be to eliminate terrorist bases. Gates said less lofty objectives may be unavoidable, simply because the country is so poor.
"If we set ourselves the objective of creating some sort of Central Asian Valhallah over there," Gates said, referring to the mythic haven of purity, "we will lose because nobody in the world has that kind of time, patience or money to be honest."
Gates' prediction comes as President Barack Obama considers his options for a drawdown of troops in Iraq. The Pentagon is preparing various scenarios for winding down the war, including a plan that would cease U.S. involvement in combat within 16 months. Gates said military planners are looking at later dates as well and are prepared to brief Mr. Obama on all his options and the their associated risks.
Mr. Obama planned to meet on Wednesday with the service chiefs.
"I believe the president will have had every opportunity to hear quite directly from his commanders about what they can accomplish and what the attendant risks are under different options," Gates said.
It was his first hearing since Mr. Obama took office and lawmakers were eager to hear details about how the administration plans to turn around the war in Afghanistan.
"It is complex," added McCain, the top Republican on the Armed Services Committee. "It is challenging. And I don't see frankly an Anbar wakening - a game-changing event - in Afghanistan, such as we were able to see in Iraq."
If we set ourselves the objective of creating some sort of Central Asian Valhallah over there, we will lose because nobody in the world has that kind of time, patience or money to be honest.
U.S. Secretary of DefenseSecurity gains made in Iraq's Anbar province are often seen as a turning point in the Iraq war.
In his prepared remarks, Gates said Afghanistan is America's "greatest military challenge" and coordination of the fight against the insurgency has been "less than stellar." He said it will take a long and difficult fight to rout militants and help develop a nation that rejects the Islamic fundamentalist Taliban and backs its own elected government.
Having recently undergone an operation to repair a damaged tendon in his left arm, Gates spoke with his arm in a sling, his coat half on.
Mr. Obama has vowed to shift military resources away from Iraq and move them toward Afghanistan and Pakistan, which he says is the central front in the struggle against terrorism and extremism. In a plan initiated during the Bush administration and endorsed by Mr. Obama, the Pentagon is planning to double the 34,000 contingent of U.S. forces in Afghanistan.
But expectations in the troubled region may have to be tempered as top military advisers focus on showing even small security gains and development progress quickly.
"That's clearly the message I'm getting is, `what are the near-term goals going to be?'" Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said when asked about Mr. Obama's agenda for Afghanistan.
While lawmakers mostly support the plan to send more troops, several Democrats have expressed the need for a clearer strategy.
Without an idea of when the commitment would end, "we tend to end up staying in different places and not necessarily resolving problems in a way that fits our national interest," said Sen. Jim Webb, D-Va., a Senate Armed Services Committee member.