The new Pentagon strategy will move more U.S. troops closer to the front lines in an effort to increase the effectiveness of Afghan forces to convince the Taliban it has no choice but to negotiate an end to the 16-year war.

"The fighting will continue to be carried out by our Afghan partners, but our advisers will accompany tactical units to advise and bring NATO fire support to bear when needed," Defense Secretary Jim Mattis told senators Tuesday.

"Make no mistake, this is combat duty, but the Afghan forces remain in the lead for the fighting," Mattis said in his opening statement the Senate Armed Services Committee.

Mattis outlined a plan to send 3,000 addition combat advisers to join Afghan units that have no advisers, and to allow all U.S. troops to go along on offensive missions against the Taliban, not just to provide advice but to call in airstrikes if necessary.

The additional troops would bring to 14,000 the number of U.S. combat forces in Afghanistan, along with 6,800 NATO and coalition troops and 320,000 Afghan National Security Forces.

"From these numbers you can see the Afghan forces remain the main effort and we are supporting them, not supplanting or substituting our troops for theirs," Mattis said.

The strategy is a major departure from the Obama administration policy, which did not allow the U.S. to take part in offensive operations against the Taliban.

The Trump plan also has no end date, which is designed to send a message to the Taliban that they cannot wait until the U.S. pulls out, and must negotiate a peace agreement.

"War is principally a matter of will, and the international community is making clear that it will stand alongside the Afghans committed to this fight," Mattis told the committee.

Mattis said the new strategy will greatly increase the combat effectiveness of Afghan forces, and U.S. and NATO airpower will make sure Afghan fighters will always have the high ground.

"We intend to drive fence-sitters and those who will see that we're not quitting this fight to reconcile with the Afghan national government," Mattis said. "Our goal is a stabilized Afghanistan, achieved through an Afghan-led, Afghan-owned peace process," Mattis said.

"This will drive them to the reconciliation that will end this war."

Joint Chiefs Chairman Gen. Joseph Dunford testified that he believed the Obama administration withdrew its advisory support for the Afghan military "prematurely," before Afghan forces were capable of carrying on the fight on their own.

But he predicted the U.S. would be able to draw down the number of troops eventually.

"In my judgment, it will be a few years, but in my judgment the Afghans will get to the point where, with a much lower level of support, they will be capable of actually securing Afghanistan," Dunford said.

A skeptical chairman Sen. John McCain opened the hearing by complaining that nine months after President Trump took office, the committee had not been fully briefed on the new plan. He also complained that neither Dunford nor Mattis submitted written answers to questions, calling the lack of communication "very disturbing." McCain later left the hearing without asking any questions of the witnesses.