Liberal Democrat foreign affairs spokesman Sir Menzies Campbell has called for an end to the US and UK "occupation" of Iraq.
But he stopped short of calling for a date for the withdrawal of troops.
He also said Britain needs an ethical foreign policy, stressing human rights and multilateralism.
In his speech to the Liberal Democrat annual conference, Sir Menzies said the Iraq war was illegal and conducted on a "false prospectus".
"The threat was manufactured not in the sands of Iraq, but in the corridors of Whitehall," Sir Menzies told delegates on the first day of the party's conference in Blackpool.
He also attacked the "inexcusable failure" to plan for post-war Iraq, which is illustrated by a "terrible daily carnage" in the country.
"Two and a half years on, we must begin to bring this occupation to an end," Sir Menzies said.
Last year the party passed a motion calling for the withdrawal of troops by the end of the year, but Sir Menzies did not name a date for withdrawal in his speech.
Sir Menzies also spoke about the challenges posed by globalisation, which he said could only be met by the co-operation and multilateralism that characterised the period immediately after the Second World War.
Other debates on Monday focus on the future of Europe, the control of small arms, NHS dentistry, mental health and international development in the form of the Millennium Development Goals.
On Sunday, Charles Kennedy began the party's conference by denying a newspaper story that he was planning to stand down after the next election.
He hailed this year's Lib Dem gathering as an "historic conference", coming in the wake of a successful election which left it with its greatest number of MPs since 1927.
The party was determined to take on the government over the threat to civil liberties posed by the government's anti-terror legislation, he said.
Lib Dem campaigns chief Lord Razzall said the party was in a "celebratory" mood and said one of its challenges over the next four years was to make voters think of Mr Kennedy as a future prime minister.