Five years later at a campaign stop on Tuesday, a Liberal MP for Toronto, Bob Rae, accused the prime minister of plagiarism.
Mr Harper's 2003 speech had been made almost word-for-word two days before in Canberra by his former Australian counterpart, John Howard, he said.
To prove the allegation, portions of the speeches were played side by side.
"In the interests of world peace and regional security... The community of nations required Iraq to surrender," Mr Howard said in his speech.
"In the interests of peace and regional security... The community of nations required Iraq to surrender," Mr Harper said two days later.
Mr Rae said the discovery was made by accident almost two months ago by Liberal strategists who were looking for similarities between Mr Harper's government and that of US President George W Bush.
The Liberals said they did not release the information until Tuesday because they had been waiting to receive a videotape of Mr Howard's speech from Australia.
Liberal leader Stephane Dion said Mr Harper ought to be "expelled" by his party, and that the context in which the plagiarism had occurred made it "even worse".
"It's about Stephen Harper saying that Canada should go with the war in Iraq," he said.
"He's unable to choose his own words," he added. "Canadians want their country [to] speak with its own voice on the world stage."
'Pressed for time'
Conservative campaign spokesman Kory Teneycke initially refused to discuss the allegations of plagiarism.
Mr Harper has been depicting himself as honest ahead of the election
"I'm not going to get into a debate about a five-year-old speech that was delivered three parliaments ago, two elections ago," he told the CBC.
But eventually Mr Lippert, who describes himself as an expert in intellectual property, apologised and announced his resignation.
"Pressed for time, I was overzealous in copying segments of another world leader's speech," he said in a statement.
"Neither my superiors in the office of the leader of the opposition nor the leader of the opposition was aware that I had done so."
The BBC's Lee Carter in Toronto says the revelation comes during an election campaign that has focused heavily on leadership, with Mr Harper depicting himself as honest and dependable, contrasting himself to Mr Dion, who has been criticised for poor leadership and communication skills.
The two men are to face each other shortly in two vital televised debates - one in French on Wednesday and another in English on Thursday.
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