But the New Democrats and Liberals continued to express wary skepticism about the government's conduct of the deployment, which has seen one special forces soldier killed in a friendly fire incident with northern Iraq's Kurdish forces.
Nicholson made the call for unity in a speech to several dozen Ottawa-based foreign diplomats who were invited to Foreign Affairs headquarters. He emphasized the threat that the Islamic State in Iraq and the Levant (ISIL) directly poses to Canada and its allies.
The speech was Nicholson's first in his new portfolio and came a day after Prime Minister Stephen Harper announced he will ask Parliament next week to extend and expand the military mission.
The NDP and the Liberals oppose the military mission, saying humanitarian aid is preferable.
Nicholson said he was "tremendously proud" of the decision to extend the mission and "the moral clarity" that Harper brings to the matter.
"I can also tell you that he will ask all parties to come together as Canadians," Nicholson said. "He will ask them to support our government's operation to degrade and destabilize this gang of thugs."
NDP foreign affairs critic Paul Dewar said his party wants to support civilians at risk from ISIL's advances, but he accused the government of not being transparent about what the military mission entails.
The Canadian Forces have deployed up to 69 special forces soldiers as advisers. Last fall's parliamentary motion ruled out using ground troops for combat operations, but since then it has emerged that those special forces have engaged ISIL in firefights. In a separate incident now being investigated, a Canadian was killed when Kurdish Peshmerga fighters mistook him for an enemy fighter.
"There is so little definition as to what our role is there, our strategy is," said Dewar. "They have to explain how it is that Canada has ended up on the front lines of this war when we weren't involved in the original 2003 invasion of Iraq."
Joyce Murray, the Liberal defence critic, said she agrees that ISIL is a threat to Canada and the world and that the country has a role to play in fighting it.
"We cannot speculate about a motion that has not been presented to Parliament," she said in an email.
"What we do know is that this prime minister has not been open and honest about what our troops are mandated to do in Iraq."
Nicholson said Canada must expand the mission because ISIL poses a continuing threat that will grow if it's not checked.
"ISIL is not dead yet," he said. "Clearly ISIL remains in control of a considerable amount of territory. It has the power to hurt, not merely in Iraq, but regionally and indeed elsewhere in the world."
He said Canada is in the fight because the militants have threatened the country directly.
Nicholson said the coalition is protecting people from what he called unspeakable atrocities.
"The plight of innocent civilians — women, children and religious minorities — is well known. All of us can be proud that we are acting boldly to protect them against ISIL's brutality."
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