Harper was responding to question period attacks by both the NDP and Liberals, who say the Conservatives misled people about the extent to which Canada's special forces troops would be involved in combat in northern Iraq.
At the outset, Harper and top military commander Gen. Tom Lawson made it clear the elite trainers would not accompany Kurdish fighters into battle, but would instead stay behind the lines to give advice and instruction.
In a statement last week, Lawson said the mission had since "evolved," but the prime minister brushed that contradiction aside Tuesday, saying it's clear the opposition parties don't support the mission.
"Once again it is the leader of the NDP who is caught in his own contradictions," the prime minister said during question period.
"Canadian troops, under their commanders, are executing exactly the mission that Canadians have given them, that this Parliament has given them, that Canadians expect. They are advising, they are assisting.
"Guess what, if fired upon they are going to shoot back; and if they kill some of the ISIL terrorists, Canadians are going to support that, no matter what the New Democrats think."
But NDP Leader Tom Mulcair says the issue is not self-defence, but whether the public can trust Harper's statements and assurances about the mission.
"The prime minister gave intentionally misleading answers," said Mulcair. "Canadians want to know why their prime minister, on something this important, did not tell the truth to Canadians."
The Liberals also attempted to drive a wedge between Harper and Defence Minister Rob Nicholson over the question of whether special forces were required from the outset to accompany Kurdish fighters to the front.
The debate happened as an Iraqis news agency reported that Lawson was in Erbil and met with high-level Kurdish officials. Canadian special forces are based in the northern city.
BasNews reported that the defence chief met with Masrour Barzani, chancellor of the Kurdistan Region Security Council, on Sunday.
The trip was not publicize by National Defence until late Tuesday, and that will likely raise more political questions about what was said, especially in light of the fact the combat mission will be up for renewal in April.
While in the region, Lawson also visited the CF-18 fighter contingent in Kuwait and HMCS Fredericton, which is deployed as part of NATO's Operation Reassurance.
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