Lawmakers also heard from Mullen, who warned that the number of casualties could increase as counterinsurgency operations became more focused.
"Although we must expect higher Alliance casualties in coming months as we dedicate more U.S. forces to protect the population and mentor the ANSF [Afghan National Security Forces], our extended security presence must -- and will -- improve security for the Afghan people and limit both future civilian and military casualties," Mullen said in his prepared remarks to the Senate. "I believe that progress in Afghanistan and Pakistan will be gradual, and sometimes halting. Yet I believe we can succeed."
To complement the growth in U.S. military forces -- the first of which will begin to arrive within two to three weeks -- the number of civilian positions in Afghanistan will increase to 974 by early next year, said Secretary of State Hillary Clinton. But she said resources for those civilian personnel "will be a challenge."
Like Gates, Clinton also stressed on the need to unite on the issue.
"We will not succeed if people view this effort as the responsibility of a single party, a single agency within our government, or a single country," Clinton said in her testimony. "We owe it to the troops and civilians who will face these dangers to come together as Americans -- and come together with our allies and international partners -- to help them accomplish this mission."
Nearly all three were united on the question of whether this is the last chance for the United States to get it right.
Mcain argued on "Good Morning America" today that setting a timeline for withdrawal will only allow the Taliban to regroup and emerge stronger when U.S. forces leave Afghanistan.
"I support the president's decision to have a properly resourced counter insurgency strategy," McCain told "GMA's" Robin Roberts. "My only difference -- and it is a significant difference -- is setting a date for return. Dates should be determined by success on the ground, not by the calendar."
The top U.S. commander on the ground, Gen. Stanley McChrystal, said today he supports the timeline and argued that it is not absolute.
"I'm absolutely supportive of the timeline," McChrystal said in an address to his commanders today. "The 18 months timeline, however, is not an absolute. It's not an 18 months and everybody leaves. The president has expressed on numerous occasions a long-term strategic partnership with Afghanistan and that includes all manners of assistance."
"If the Taliban melted away and left the people alone for 18 months or longer, in fact, what would happen in my view is the capacity of the Afghan government through its security forces but also local governance and development would make it much more difficult for insurgents returning to contest that," McChrystal added.
McCain, however, said the Taliban will be inclined to make other arrangements and return when U.S. forces start withdrawing.
"I trust his [McChrystal's] judgment enormously but I also understand that both our enemies and our friends alike hear the message that we are going to be leaving at a certain date. That was unnecessary," McCain said. "I'm confident we can succeed, but when you tell your enemies that there's a date that you're going to start leaving... it emboldens your enemies and dispirits your friends."