The Liberal leader said Harper has made no attempt to justify a combat mission or to foster an all-party consensus on the issue.
Indeed, Trudeau said the prime minister seems to relish casting himself as the lone, macho, tough guy among federal leaders.
"Unlike prime ministers for decades before him, Mr. Harper has made no effort to build a non-partisan case for war," Trudeau told a conference hosted by Canada2020, a progressive think tank.
"Instead, he dares us to oppose his war, staking out not moral territory but political territory."
Trudeau said he hasn't yet decided whether Liberals will support a combat mission, but made it clear his inclination is to have Canada stick to a non-combat role in the U.S.-led fight against the militant Islamic State of Iraq and the Levant.
"Why aren't we talking more about the kind of humanitarian aid that Canada can and must be engaged in, rather than trying to whip out our CF-18s and show them how big they are? It just doesn't work like that in Canada."
Harper is expected to announce Friday that Canada will deploy CF-18 fighter jets to take part in air strikes against the militants in Iraq.
Conservatives quickly struck back at Trudeau's comments.
"Mr. Trudeau's comments are disrespectful of the Canadian Armed Forces and make light of a serious issue," said Jason MacDonald, the prime minister's spokesman.
"Our involvement in the fight against ISIL is, and has been, motivated by a desire to do our part in fighting a group that has made direct terrorist threats against Canada and Canadians, in addition to carrying out atrocities against children, women, and men in the region. As the prime minister has said, we take that seriously and will do our part."
Contrary to Trudeau's contention, MacDonald said Canada already ranks as one of the most generous humanitarian aid donors in Iraq.
Employment Minister Jason Kenney said making a "juvenile high school joke" about the global campaign to combat a "genocidal terrorist organization" raises more questions about Trudeau's judgment.
Although he shares many of the same concerns expressed by Trudeau, NDP Leader Tom Mulcair also waded in, accusing Trudeau of making "childish jokes" while Harper refuses to answer questions about Canada's role in Iraq.
"I think Canadians are going to judge both rather harshly," Mulcair predicted.
The Liberals supported the government's decision to send a small number of special forces members into Iraq for 30 days to help train Kurdish soldiers who are battling ISIL extremists. But whether that support will extend to a combat role is doubtful.
While he acknowledged that Canada has a duty to help deal with the "global security threat" posed by ISIL, Trudeau questioned whether deploying "a handful of aging war planes" is the best contribution Canada can make.
"I think Canadians have a lot more in them than that. We can be resourceful and there are significant, substantial, non-combat roles that Canada can play," he said, suggesting Canada could do more to provide strategic airlift, training, medical support and humanitarian aid for the thousands of displaced Iraqis.
If Harper wants Liberal support for a combat mission, Trudeau said he'll have to make the case "openly and transparently, based on clear and reliable, dispassionately presented facts."
Trudeau reminded the conference that Harper supported the 2003 U.S.-led war in Iraq, which was sold to the public "with overheated, moralistic rhetoric that obscured very real flaws in the strategy and the plan to implement it."
He accused Harper of taking the same approach to the current conflict, talking about "the nobility of combat," while refusing to tell Canadians what military support he's offered the Americans, how long a combat role would last or how helpful the CF-18s can actually be.
"Mr. Harper is intent on taking Canada to war in Iraq. He needs to justify that. He has not made the case for it. He hasn't even tried."
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