Barack Obama absolved CIA officers from prosecution for harsh, painful interrogation of terror suspects, even as his administration released Bush-era memos graphically detailing and authorising the grim tactics.
The techniques included slamming detainees against walls, waterboarding them and keeping them naked and cold for long periods.
Human rights groups and many Obama officials have condemned such methods as torture.
In releasing the documents yesterday, the most comprehensive accounting yet of interrogation methods that were among the Bush administration's most closely-guarded secrets, Mr Obama said he wanted to move beyond "a dark and painful chapter in our history".
Past and present CIA officials had pressed unsuccessfully for more parts of the four legal memos to be kept secret and some critics argued the release would make the United States less safe.
Michael Hayden, who led the CIA under George Bush, said officers would now be more timid and allies more reluctant to share sensitive intelligence.
"If you want an intelligence service to work for you, they always work on the edge. That's just where they work," Mr Hayden said.
Now, he said, foreign partners would be less likely to co-operate with the CIA because the release showed they "can't keep anything secret".
Meanwhile human rights advocates said Mr Obama should not have assured the CIA that officers who conducted the interrogations would not be prosecuted if they used methods authorised by Bush lawyers in the memos.
But Mr Obama said in a statement: "Nothing will be gained by spending our time and energy laying blame for the past."
The Bush administration memos describe the tough interrogation methods used against 28 terror suspects, the fullest and now complete government accounting of the techniques. They range from waterboarding - simulated drowning - to using a plastic neck collar to slam detainees into walls.
Other methods were more psychological. One technique approved but never used, involved putting a detainee who had shown a fear of insects into a box filled with caterpillars.
The documents also offer justification for the tough tactics.
A May 30 2005, memo says that before the harsher methods were used on top al-Qaida detainee Khalid Sheikh Mohammed, he refused to answer questions about pending plots against the US.
"Soon, you will know," he told them, according to the memo.
It says the interrogations later extracted details of a plot called the "second wave" to use east Asian operatives to crash a hijacked airliner into a building in Los Angeles.
Terror plots that were disrupted, the memos say, include the alleged effort by Jose Padilla to detonate a "dirty bomb" spreading nuclear radiation. The "dirty bomb" charges were eventually dropped.
Even as they exposed new details of the interrogation programme, Mr Obama and US attorney general Eric Holder offered the first definitive assurance that the CIA officials involved were in the clear as long as their actions were in line with legal advice at the time.
Mr Holder went further, telling the CIA the government would provide free legal representation to its employees in any legal proceeding or congressional investigation related to the programme and would repay any financial judgment.
"It would be unfair to prosecute dedicated men and women working to protect America for conduct that was sanctioned in advance by the justice department," Mr Holder said.
Mr Obama said in his statement and a separate letter sent directly to CIA employees that the nation must protect their identity "as vigilantly as they protect our security".
Current CIA director Leon Panetta said in a message to his employees: "CIA responded, as duty requires."
Some parts of the memos were blacked out, and Mr Panetta had pushed for more redactions, according to a government official.
The CIA has acknowledged using waterboarding on three high-level terror detainees in 2002 and 2003, with the authorisation of the White House and the justice department.
Mr Hayden said waterboarding has not been used since, but some human rights groups have urged Mr Obama to hold CIA employees accountable for what they and many Obama officials say was torture.