Defense Secretary Jim Mattis has dropped several big hints about what will be in his new strategy for Afghanistan, an almost 16-year war he told Congress this week that the U.S. is "not winning." According to one source, beyond tactical moves in the country, the plan involves focusing on terrorists just over the border in Pakistan.

In his public statements before four congressional committees, Mattis promised to correct the mistakes of the Obama administration, in particular by backing up beleaguered Afghan government troops by restoring robust NATO air support for ground operations against the Taliban.

"The revised Afghanistan strategy with a new approach will be presented to the president for his approval in the coming weeks," Mattis said.

As part of that new approach, Mattis is considering sending 3,000 to 5,000 additional U.S. advisers and trainers to reinforce the NATO training mission designed to get the Afghan Security Forces to a point where they can handle the fight alone.

"Our overall mission in Afghanistan remains the same; to train, advise and assist the Afghan forces so they can safeguard the Afghan people and terrorists find no haven in Afghanistan for attacking us or others."

But Mattis argues that to be successful, any new strategy must extend beyond the borders of Afghanistan.

"We're going to have to look at a more regional strategy, one that takes into account Afghanistan as part of South Asia, not look at it in isolation," Mattis told Congress.

What that means, according to a senior Pentagon official who spoke under condition of anonymity because the strategy has not been finalized, pressuring Pakistan to go after the Haqqani network, a shadowy wing of the Taliban that operates across the border in Pakistan, and which the U.S. and Afghanistan suspect are secretly supported by Pakistan's military intelligence service, the Inter-Services Intelligence or ISI.

The group is suspected of being behind Afghanistan's deadliest attack, a massive truck bomb that killed 150 people near the German embassy in Kabul May 31.

The Taliban denied responsibility for the suicide bombing, but Afghan intelligence said it believes the attack was designed to disrupt the peace process.

"We have to give Pakistan an incentive to move against the Haqqani network," said the official familiar with Mattis' thinking. "That's what the strategy is."

Mattis said his strategy will be "one that marries itself to reality," and ultimately has to lead to the warring parties sitting and making peace.

"We're not looking at a purely military strategy," Mattis testified this week. "It's got to be one that leads to reconciliation. All wars come to an end."

While Mattis is widely respected as a legendary commander and a master tactician, he does face some skepticism from a war-weary Congress.

On Wednesday before the Senate Appropriations Subcommittee on Defense, Sen. John Tester, D-Mont. questioned whether the "graveyard of empires" was a war anyone could win.

"Fifteen years in, and I always think back to the fall of the Soviet Union. And some people say the reason that they went down as they did was their involvement in Afghanistan," Tester said.

"The question is, is it achievable?" Tester asked. "This country's been at war for God knows how many centuries."

"Yes, it is achievable," Mattis replied. "But the international community is going to have to hold with it and when we reduce, we reduce based on conditions on the ground, not on arbitrary timeline."