The two-hour debate, held in Toronto and hosted by Munk Debates, covered a number of foreign policy issues including Canada's ISIS mission, the Syrian refugee crisis and anti-terrorism legislation.
Harper's record was the focus of attacks for most of the evening. But it was the issue of Bill C-24 that sparked the liveliest verbal confrontation in the debate, which was mostly void of the typical exchanges of prior contests in which leaders frequently talked over one another.
Trudeau said the Liberal Party was worried that the first response by the government against individuals convicted of certain crimes was not to keep them in jail, but that they be subject to a two-tiered citizenship. Trudeau said it's worrisome and a slippery slope, that a prime minister has the ability to revoke citizenship for people.
But Harper shot back that Canada has already revoked citizenship of war criminals. The Conservative government, has moved to revoke the citizenship of those linked to extremist activity, including three members of the so-called Toronto 18 bomb plot.
"Why would we not revoke the citizenship of people convicted of terrorist offences of this country?" Harper asked.
"A Canadian is a Canadian is a Canadian," Trudeau responded. "And you devalue the citizenship of every Canadian in this place and in this country when you break down and make it conditional for anybody."
A back and forth between the two leaders ensued, with Trudeau insisting that those types of individuals, regardless of their crimes, should stay in jail.
"This country has every right to revoke the citizenship of an individual like that," Harper said.
Trudeau and NDP Leader Tom Mulcair have vowed to repeal Bill C-24, dubbed the Strengthening Canadian Citizenship Act, which allows a dual national to be stripped of citizenship if convicted of offences related to spying, treason or terrorism.
On the issue of ISIS, Harper defended Canada's military mission, which involves airstrikes and training local ground troops. Harper said that ISIS unchecked has the potential to slaughter millions, and if left unchallenged it would set up terrorist base camps in Iraq and Syria that could threaten Canada.
"We have a very clear reason for being there. Why we would abandon this mission is a question that goes begging," Harper said.
Mulcair, who has said his government would bring back all the troops from the Iraq mission, said more than 60 countries are involved in the mission, but only 12 involved in combat.
"It is important to remember that this is not a NATO mission. This is not a United Nations mission," he said.
Trudeau suggested that Harper never met a military mission in the Middle East he didn't like, including supporting the 2003 Iraq invasion. But he also rejected Mulcair's approach, saying he doesn't want to be in any fight.
Instead, Canadians should be focused on training local forces so they can defeat ISIS on the ground, he said.
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