Opposing Canada's mission against the Islamic State never posed much of a dilemma for New Democratic Party Leader Tom Mulcair. His position is consistent with everything his party stands for.

It was never as cut and dried for Justin Trudeau. The Liberal Leader's popularity has declined in sync with the rising perceived threat from jihadi terrorism. His soft-on-terrorism image was not helped by his decision to oppose Canada's initial participation in air strikes against the Islamic State in Iraq.

As the clock ran down on the initial mission, with its extension by the Harper government all but certain, there was an opportunity for Mr. Trudeau to recalibrate. Liberals are not against all wars, just – as a future U.S. president would dub the 2003 invasion of Iraq – the "dumb" ones. Former prime minister Jean Chrétien's government kept Canada out of that disaster, but signed us up for combat in Afghanistan. With U.S. President Barack Obama leading the anti-Islamic State military coalition, and a majority of Canadians supporting our role in it, Mr. Trudeau faced a tough decision.

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In the end, Stephen Harper made it easy for him. The Prime Minister's motion to extend Canadian air strikes into Syria is fraught with all the legal and moral ambiguity Liberals need to credibly oppose it. Not only does striking at Islamic State targets in Syria without the express consent of the Syrian government violate international law, it may even make Canada complicit in propping up Bashar al-Assad, Syria's vile President.

"This is a man who has used chemical weapons on his own citizens and whose regime is responsible for torturing and killing many more innocent people than even ISIL," Mr. Trudeau said in the House of Commons, using an alternative acronym for the Islamic State. "We cannot support a mission that could very well result in Assad consolidating his grip on power in Syria."

Mr. Trudeau is right about Mr. Assad. But he might more properly take up the matter of the Syrian leader's staying power with Mr. Obama. The U.S. President no longer asserts that Mr. Assad has "lost all legitimacy" and "must go." He has apparently concluded that the Syrian butcher's departure must wait in order to keep the anti-Islamic State forces together. These include Iraq's Shia-led military and Iranian-backed Shia militias in Iraq, who support Mr. Assad, a member of the Shia-aligned Alawite sect.

Coalition air strikes against Islamic State targets in Syria could be seen to be doing Mr. Assad's bidding. As coalition bombs seek to contain Islamic State forces in Syria, Mr. Assad is free to continue his assault on non-jihadi opposition groups seeking his ouster.

Mr. Obama's plan, long in coming and painfully slow to materialize, is to train a new army of anti-Assad Syrian rebels to take on the Islamic State (and perhaps eventually Mr. Assad himself.) But the President's own former ambassador to Syria, Robert S. Ford, says this will be "too little, too late." Not only will this force be outnumbered by Islamic State jihadis, "the plan will further split the moderate armed opposition and will do nothing to counter the Islamic State's biggest recruitment tool – the Assad regime's brutality."

In short, Mr. Assad's defeat is ultimately a prerequisite for defeating the Islamic State. But figuring out how to to square this circle is the Obama administration's problem. It is largely responsible for the chaos in Syria. Mr. Obama dithered over whether to aid the moderate rebels against Mr. Assad, in the end doing nothing. The Islamic State filled the resulting vacuum, using Syria as its base for capturing territory there and in Iraq. Until the "geopolitical Chernobyl" in Syria is capped, retired general David Petraeus said last week, "it is going to continue to spew radioactive instability and extremist ideology over the entire region."

Canada can do little good in Syria, however. If additional air strikes are needed to contain the Islamic State's control of territory there, U.S. forces are more than equipped to do so. Better to focus our limited resources on targets in Iraq, where we are already playing a meaningful role aiding the Kurdish peshmerga.

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There are those who say that Canada should stay out, period. That taking sides in what is really a regional conflict between sectarian forces will have unintended consequences and come back to bite us. That bombing the Islamic State will only enhance its appeal among disaffected Canadian youth.

Most Liberals would not typically agree with that. But by extending the mission into Syria, Mr. Harper has given Mr. Trudeau a good reason to oppose it. The Prime Minister, no doubt, prefers it that way.