In a contentious House of Representatives committee hearing on Wednesday, Powell continued the administration counter-offensive against claims Bush tailored intelligence to take America into an unnecessary war.
More used to a fawning welcome by senior representatives, Powell faced several harsh critiques on Iraq, and clashed with outspoken New York Democrat Gary Ackerman.
"Truth is the first casualty of war, I would contend that the truth was murdered before a shot was fired," Ackerman told the House International Relations panel.
"This administration lacks credibility with Congress, the American people and the international community."
Controversial UN presentation
Powell snapped back, "the truth was not murdered Mr Ackerman, nobody shaped it, nobody told the intelligence community what to say".
He defended his presentation to the United Nations Security Council on 5 February last year, which argued that Saddam was deceiving UN weapons inspectors and pursuing banned weapons programmes.
"Truth is the first casualty of war, I would contend that the truth was murdered before a shot was fired. This administration lacks credibility with Congress, the American people and the international community"
"I went into that briefing believing there were stockpiles, that there were weapons there, that we expected to find them.
"It was not a question of we knew nothing was there and we lied about it, what we did was we presented the facts that our intelligence community provided to us, nothing more nothing less.
"I did not go before the UN and tell anything but the truth as we knew it at the time."
Powell also defended Bush who last week announced an independent probe into US intelligence gathering on weapons of mass destruction.
"I don't think we have anything to be apologetic about and under no set of circumstances do I believe that anybody in America should think that the president cooked the books or in some way tried to mislead them."
Powell argued that although no weapons of mass destruction had yet been discovered in Iraq, there was proof that Saddam Hussein had been developing long-range missile systems.
The former general also uncharacteristically lost his patience, rounding on a House of Representatives aide he said was casting doubt on his testimony.
"Are you shaking your head for something young man back there?" Powell said, directing his remarks to man at the back of the hearing room. "Are you part of this proceeding?"
Bush under pressure
"It was not a question of we knew nothing was there and we lied about it, what we did was we presented the facts that our intelligence community provided to us, nothing more nothing less"
Colin Powell on his presentation to the UN before the Iraq war
"I seldom come to a meeting where I am talking to a congressman and I have people giving editorial comment by head shakes."
In a television interview on Sunday, President Bush insisted the Iraq invasion was a "war of necessity" amid growing signs that the failure to find unconventional weapons has hurt his credibility before the November US presidential election.
He told NBC television that Saddam "had the capacity" to make weapons of mass destruction.
Bush suggested Saddam might have destroyed his arsenals, hidden them or transported them to a nearby country.
Democratic Party candidates battling for the right to challenge Bush in the general election in November, have accused Bush of leading Americans into a quagmire in Iraq, and of hyping the threat posed by Saddam Hussein.