It was a mission to buy time. The advances of Islamic State fighters had to be stopped before they took more territory in Iraq. A U.S.-led coalition, including Canada, would use air strikes to push back black-clad jihadis, allowing Kurdish fighters and the Iraqi army to regroup.

That was last fall, when the Commons first debated a military mission against the Islamic State. Six months later, Canada is signing up for another year, with no confidence it will be enough. This time, the strikes will be expanded from Iraq, where the coalition had a hope for stability, to Syria, where there's none.

Now, there's a debate in Parliament over extension and expansion of the mission with a vote coming Monday, but the outcome is no mystery: In the Commons, and in Canada, a majority believe this extremist group must be thwarted.

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But even for those who feel the Islamic State must be confronted with force, there can be no doubt this extension marks a step into a murkier war.

Striking the Islamic State in Syria may make tactical sense, but it threatens to yield byproducts, such as aiding the bloody Assad regime or competing extremist factions.

Some raise doubts that it can be justified under international law. And the expansion to Syria, and one-year extension, underlines that the coalition strategy to buy time in Iraq is not working as planned.

The end game remains unclear. Prime Minister Stephen Harper's government boasts about cutting through those qualms with "moral clarity" and an unwavering mission to get the bad guys.

How did we get here?

Two things have evolved: the domestic political environment, and Mr. Harper's view of strikes in Syria.

Six months ago, Mr. Harper was cautious about how a war-weary public would react. He stressed limits on the Canadian mission. But polls have shown majority support. Both NDP Leader Thomas Mulcair and Liberal Leader Justin Trudeau opposed the mission – handing him sole ownership of a popular policy. He was emboldened.

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In the meantime, military assessments convinced him that expanding strikes to Syria was the right tactical decision, according to aides. There were signs Islamic State fighters were moving equipment from Iraq to their "western front" in Syria, they said, along with a decreasing number of targets in Iraq. While several European countries flew strike missions in Iraq, only the United States and Arab allies did so in Syria. And the U.S. military, according to Defence Minister Jason Kenney, wanted Canadian precision munitions in Syria.

Over six months of air strikes in Syria, the Assad regime hadn't fired at U.S. planes. There was an understanding Syria accepted the strikes.

What will change?

In one sense, expanding air strikes across the border into Syria changes nothing. "If the purpose is to degrade and destroy Islamic State, then it really doesn't matter where you're bombing them," said Ben Connable, a senior international policy analyst with the Rand Corp. The border has effectively been erased, so it makes tactical sense to hit them on both sides.

The broader strategic picture is more cloudy. Striking the Islamic State in Syria could aid the regime of Bashar al-Assad. There are reports the strikes are already helping rival jihadis in the al-Qaeda-affiliated Nusra Front. There are hundreds of other factions, and it's unclear how air strikes will affect the chaos.

Perhaps the coalition is willing to risk that to weaken the Islamic State's hold on Iraq. But the mission in Iraq has turned cloudy, too. Air strikes have helped thwart the Islamic State's advances, but buying time has not gone as planned. There are no longer realistic hopes Iraq's army will be rebuilt quickly, Mr. Connable said. Instead, Shia militias, advised by Iran, are the ground forces attacking the Islamic State. And that risks exacerbating the Shia-Sunni sectarian rift.

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Moral clarity or murky reality?

The Conservatives have met those concerns with assertions that Canada must do more than wield pious words against extremists. "We must act with compassion, with strength and with moral clarity," Foreign Affairs Minister Rob Nicholson said this week.

That's what the Conservatives tout in this debate: moral clarity. Mr. Harper laughed off concerns that air strikes in Syria might violate international law, joking that the Islamic State won't take us to court. The government's real legal argument – that Syria doesn't have control of part of its territory so others may attack threats emanating from there – is contested by experts, but in practice Syria doesn't contest it.

The moral arguments, of preventing atrocities, don't hold together as well for Syria. Roland Paris, the University of Ottawa's chair in international security, noted Canada won't commit to protecting Syrians now, after years of bloody civil war.

But Mr. Harper's government has a straightforward argument few in Canada contest: Islamic State militants are bad guys who should be stopped. Opponents such as Green Party Leader Elizabeth May, however, have raised the fear that intervention will do more harm. She noted Canada joined Libyan air strikes with good intentions, to protect civilians, then shifted to a goal of regime change, arguing nothing could be worse than Moammar Gadhafi. "I don't think there's any question we made things much, much worse," she said.

What's the end game?

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Like the coalition as a whole, Canada's end game is unclear. Mr. Kenney described the goal as defeating the Islamic State, but then reverted to saying the aim is to degrade it. It's not clear how much degrading is enough.

It's possible allies on the ground could coalesce. Perhaps Iraq can reconcile its sectarian rift. Perhaps Iran-backed Shia militias will defeat Islamic State, leaving different problems. But Mr. Connable said the coalition might feel its only option is to continue air strikes for years. That has some fearing the coalition, and Canada, could be drawn in further – to commit ground combat troops.

But Mr. Harper's government insists Canada will decide when to opt out. The exit strategy for Canada, Mr. Kenney said Thursday, is simple: The government can at any point declare the mission over and put Canadian Forces personnel on a plane home.