Barack Obama has offered his most vigorous public defence of drone strikes, describing them as legal, effective and necessary.
The US president's remarks came as he sought to move America beyond the war effort of the past dozen years, defining a narrower terror threat from smaller networks and home-grown extremists, rather than the grandiose plots of Osama bin Laden's al Qaida.
"Neither I, nor any president, can promise the total defeat of terror," he said in a speech at Washington's National Defence University.
"What we can do - what we must do - is dismantle networks that pose a direct danger and make it less likely for new groups to gain a foothold, all while maintaining the freedoms and ideals that we defend."
Mr Obama also implored Congress to close the much-criticised Guantanamo Bay detention centre in Cuba and pledged to allow greater oversight of the controversial unmanned drone programme. But he plans to keep the most lethal efforts with the unmanned aircraft under the CIA's control.
It is an awkward position for the president, a constitutional lawyer, who took office pledging to undo policies that infringed on Americans' civil liberties and hurt the US image around the world. "Now is the time to ask ourselves hard questions - about the nature of today's threats and how we should confront them," Mr Obama said.
His address came amid increased pressure from Congress on both issues. A rare coalition of bi-partisan politicians has pressed for more openness and more oversight of the highly-secretive targeted drone strikes.
The president cast the drone programme as crucial in a counterterror effort that will rely less on the widespread deployment of US troops as the war in Afghanistan winds down. He said he was deeply troubled by the civilians unintentionally killed.
Some Republicans criticised Mr Obama as underestimating the strength of al Qaida and for proposing to repeal the president's broad authorisation to use military force against the nation's enemies - powers granted to George Bush after the September 11 2001 attacks.
"I believe we are still in a long, drawn-out conflict with al Qaida," Senator John McCain, a top opposition Republican, said. "To somehow argue that al Qaida is on the run comes from a degree of unreality that to me is really incredible. Al Qaida is expanding all over the Middle East, from Mali to Yemen and all places in between."