The Carnegie Endowment for International Peace said on Wednesday there had been no immediate threat to the United States or the Middle East.
Its WMD study asserts there was "no convincing evidence" Iraq had reconstituted its nuclear programme.
The report adds that the nerve agents found by UN weapons inspectors in Iraq's chemical weapons programme had lost most of their lethal capability, as early as 1991.
There was greater uncertainty about Iraq's biological weapons, but that threat was related to what could be developed in the future rather than what Iraq already had, the study by the liberal-leaning think tank said.
However, the missile programme appeared to have been in active development in 2002 and Iraq was expanding its capability to build missiles with ranges that exceeded UN limits.
No stockpiles found
The US justified going to war against Iraq last year citing a threat from Baghdad's weapons of mass destruction.
Since the US occupation of Iraq, American forces hunting for weapons of mass destruction have not found any stockpiles of biological or chemical weapons or any solid evidence Iraq had resurrected its nuclear weapons programme.
It was unlikely Iraq could have destroyed, hidden, or moved out of the country hundreds of tons of chemical and biological weapons, dozens of SCUD missiles, and facilities producing chemical and biological weapons without the United States
detecting some sign of that activity, the report said.
"Administration officials systematically misrepresented the threat from Iraq's WMD and ballistic missile programs," the report said.
"There was no evidence to support the claim that Iraq would have transferred WMD to al-Qaida and much evidence to counter it"
The Carnegie Endowment for International Peace
By lumping nuclear, chemical and biological weapons together as a single threat - despite the "very different" danger they posed - distorted the cost/benefit analysis of the war.
Administration officials also insisted - without evidence - that former Iraqi President Saddam Hussein would give weapons of mass destruction to terrorists, the report said.
"There was no evidence to support the claim that Iraq would have transferred WMD to al-Qaida and much evidence to counter it," the report said, adding there was no history of any cooperation.
Prior to 2002, intelligence agencies appeared to have overestimated the chemical and biological weapons in Iraq, but had a generally accurate reading of the nuclear and missile programmes, the study said.
But from 2002 until the war in Iraq, there appeared to have been an environment of intense political pressure in which an October 2002 National Intelligence Estimate on Iraq's banned weapons was hurriedly put together.
The estimate included a high number of dissents in what was supposed to be a consensus document of the various intelligence agencies, the study said. The Pentagon created a separate intelligence office during that time.
Those factors suggested "the intelligence community began to be unduly influenced by policymakers' views," the study said.
However, Stuart Cohen - vice chairman of the National Intelligence Council - told ABC's "Nightline" on Tuesday "assertions, particularly that we had shaded our judgments to support an administration policy, were just nonsense."