“We’re now six months into this administration,” Mr. McCain said. “We still haven’t got a strategy for Afghanistan. It makes it hard for us to support you when we don’t have a strategy.”
Mr. Mattis sought to ease his concerns by hinting that some troops might be sent as an interim step before the administration’s new strategy is finalized. “There are actions being taken to make certain that we don’t pay a price for the delay,” Mr. Mattis said.
Mr. Trump has already given his Pentagon chief similar authority for Iraq and Syria.
Mr. Trump’s approach makes a sharp break from former President Barack Obama, who tightly controlled decisions on military troops, a practice that some critics complained smacked of micromanaging. The president has relaxed the rules for counterterrorism operations in Somalia and Yemen and was quick to approve the military’s plan to fire sea-launched cruise missiles at an airfield that President Bashar al-Assad’s forces used to mount a chemical weapons attack in Syria.
Proponents say that delegating the authority to the Pentagon will enable it to carry out campaigns against the United States’ adversaries without interruptions and will allow it to respond more quickly to changes on the battlefield. The risk, critics say, is the president may become too detached from developments on the battlefield and may use this approach to distance himself from a decision that could be politically unpopular.
There is no debate that the war is Afghanistan is not going well. Gen. John W. Nicholson, the commander of the American-led international force in Afghanistan, told Congress in February that the United States and its NATO allies were facing a “stalemate.”
Mr. Mattis offered a similarly sober assessment. “The Taliban had a good year last year, and they’re trying to have a good one this year,” he said. “Right now, I believe the enemy is surging.”
The main question before the administration is how to reverse the trends on the battlefield.
The military’s advice from the field has long been clear. In his February testimony, General Nicholson said he needed a “few thousand” troops.
The Pentagon later developed options to send 3,000 to 5,000 more American troops to Afghanistan, including hundreds of Special Operations forces. The reinforcements would be augmented by troop contributions by NATO nations, which American officials have begun to solicit.
But Mr. Trump had long expressed skepticism about sending more troops, and the issue has never been easy for an administration that trumpets an “America first” strategy. While Mr. Trump has vowed to defeat terrorist groups, sending more American forces to Afghanistan could cost billions of dollars, and there is no guarantee of producing a clear win.
Even as Mr. Trump has granted Mr. Mattis the authority to set troop levels, the administration’s broader strategy review has yet to be completed. Officials said the Afghan review has been broadened to include the policy toward neighboring Pakistan, particularly the question of how to prevent that country from being a haven for the Taliban and militants involved in the Afghan conflict.
That in turn has led to a discussion within the administration about what steps might be taken to mitigate Pakistan’s decades-long anxieties over India. The result is that the Afghan review has turned into a larger review of American policy toward Southwest Asia.
Mr. Mattis did not discuss the details of the review with the senators on Tuesday, but he vowed to reverse the slide.
“We are not winning in Afghanistan right now, and we will correct this as soon as possible,” he said. “It’s going to require a change in our approach from the last several years.”
Alluding to the troop reinforcement plan that has been under discussion, Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, said the United States and allies could help reduce the substantial number of casualties that Afghan forces have sustained by “more effectively advising them, both in planning operations and delivering combined arms.”
One argument that proponents have made for sending more troops is that it would enable the United States to advise Afghan units closer to the battlefield.
Asked what it would mean to win in Afghanistan, Mr. Mattis provided a definition that might have been produced by the Obama administration.
The idea, he said, would be to drive down the violence to a level that could be managed by Afghan government forces with the help of American and allied troops in training their Afghan counterparts, providing intelligence and delivering what Mr. Mattis called “high-end capability,” an apparent allusion to air power and possibly Special Operations forces.
The result, he said, would be an “era of frequent skirmishing,” but not a situation in which the Afghan government no longer faced a mortal threat.
The main purpose of the hearing was to review the Trump administration’s $603 billion military spending request, which represents a 3 percent increase over Mr. Obama’s last defense plan.
Mr. McCain and other hawkish lawmakers have described the request as far too little to carry out the military buildup Mr. Trump has advertised, a point that Mr. Mattis and General Dunford uncomfortably acknowledged.
The Pentagon officials asserted that the current spending request would help the military improve the readiness of its existing forces while 3 to 5 percent growth would be needed during the 2019 to 2023 budget years to expand the size of the military and buy new weapons.