"Nobody wants to go to war, nobody wants to see innocent people die, nobody wants to see their forces put at risk of their lives," he said, but added: "I think it was the right decision and made for the right reasons."
He said Britain hoped "right up to the last minute" to tackle the threat posed by Iraqi leader Saddam Hussein through the United Nations, but when this failed it had to act to preserve the international community's credibility.
Brown's appearance before the Chilcot inquiry in London was a political wild card just weeks before a general election expected on May 6. The conflict, which left 179 British soldiers dead, remains hugely divisive here.
Much of the British responsibility for the US-led war has been laid on Blair, who appeared at the inquiry in January, but Brown is facing damaging claims that as finance minister he failed to properly fund the armed forces.
Witnesses to the inquiry, including former defence minister Geoff Hoon, have said money was tight, and a former armed services chief alleged Friday that lives were lost because Brown ignored pleas for funding.
General Charles Guthrie told The Times: "Not fully funding the army in the way they had asked... undoubtedly cost the lives of soldiers."
However, Brown insisted he had met all requests for extra resources.
"At any point military commanders were able to ask for equipment that they needed and I know of no occasion when they were turned down for it," he said.
Brown said the war cost about eight billion pounds (12 billion dollars, 8.9 billion euros) overall, on top of an increasing defence budget.
Anti-war campaigners argue this could have been better spent, and a small group of protesters brandished a blood-soaked cheque outside the hearing venue where they noted the schools and hospitals the money would have paid for.
Yet some relatives of the dead soldiers say more should have been spent on military equipment.
After hearing Brown's first evidence sessions, Susan Smith, whose 21-year-old son Phillip Hewett died when his lightly armoured Snatch Land Rover was blown up in Iraq in July 2005, was unimpressed by his account.
"He's saying there's this much money here and there was that much money there. But he's not actually answering anything," she said.
Brown admitted he was not at some key meetings Blair held in the run-up to war, but said he was fully briefed on the intelligence that helped build the case against Saddam and on advice about the war's legality.
"I did not feel at any point that I lacked the information that was necessary, that I was denied any information that was required," he said.
"But my role in this was not to second-guess military decisions or options, my role was not to intervene in what were very important diplomatic negotiations... (it was) to make sure that the funding was there for what we had to do."
Brown admitted "regrets" about planning the reconstruction of post-war Iraq but laid much of the blame on US president George W. Bush's administration, and criticised the "neo-conservative proposition that somehow, at the barrel of a gun, overnight liberty or democracy could be conjured up".
Where Blair concluded his inquiry hearing with a defiant claim to having "no regrets" about the war, Brown paid tribute to the soldiers and Iraqi civilians who died. "The loss of life is something that makes us all sad," he said.
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