In expanding its military mission against the so-called Islamic State, Canada could become embroiled in the deteriorating situation in Syria. Here are the scenarios it could face:

Best case: Islamic State defeated

Carrying out bombing runs against Islamic State fighters will not, alone, rout the powerful and widespread Islamist extremists. However, the combination of air attacks and ground troops in Syria – along with success in driving IS forces from Iraq – could result in the conquering of Islamic State.

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Provided no other Islamist group rises to IS-type levels, the stage would then be set for the Bashar al-Assad regime to take charge of the country again. (Indeed, the on-ground troops that defeat IS forces would almost certainly include large numbers of Syrian soldiers.) In this scenario, the regime would be assisted by Iranian-backed militias – Hezbollah from Lebanon and Iraqi Shia militias – sealing Syria's borders and hampering the flow of materiel and new recruits.

In this event, after some months of calm, large numbers of refugees – those who can abide an Assad administration – would return to Syria from Turkey, Lebanon and Jordan. While this means that Canada and other coalition partners would have assisted Mr. al-Assad, an Iranian-backed dictator, in regaining national supremacy, the very best scenario would have him quickly hand over power to an interim authority that would oversee democratic elections.

Canada's mission would be completed and its forces withdrawn.

Worst case: Islamists triumph

Islamic State, the al-Qaeda-linked Nusrah Front and other Salafi-jihadi groups have shown remarkable resilience in battling Syria, including the capacity to replenish personnel and materiel from sources outside the country. Nothing short of an extensive ground campaign is likely to overcome them. In this scenario, Islamic State and other Islamist groups succeed in driving Syrian President Mr. al-Assad's forces out of Damascus and declare what they term a "caliphate" in much of the country. While the support of Hezbollah forces from Lebanon and Shia militias from Iraq make it unlikely the Assad regime will be toppled this way, this is rather the way the House of Saud came to power in Arabia.

Should it happen, the "caliphate" would function with the assistance of supporters in Saudi Arabia, Turkey, Lebanon and Jordan, and would become a well-armed base for many Sunni extremist fighters.

Mr. al-Assad and many of his fellow Syrian Alawites would flee to their homeland in the mountainous area near Latakia, where they might survive for years. Others would flee the country, adding to the refugee burden in neighbouring states. Alawites, like Shiites in general, are viewed as heretics by Islamic State and risk their lives should they remain.

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Canada's mission would have failed and it would have to decide whether all-out war against the caliphate was warranted.

Most likely: war without end

Without a ground campaign, the self-styled Islamic State will almost certainly survive and other Islamist groups will proliferate though, even together, they would not likely be able to uproot the Assad regime. In this event, the civil war against Damascus would continue and Islamic State would remain a target of the U.S.-led coalition, including Canada.

Iran's likely success against IS forces in Iraq will stiffen Mr. al-Assad's resolve and fighting, already in its fifth year, could go on interminably. As it does, Syria will go on bleeding refugees to Lebanon and Jordan where instability may well lead to violence and to attempts at overthrowing the ruling authorities.

Lebanon's Hezbollah movement, already stretched by its support for the Assad regime, could become a target by Lebanese Christian or Sunni Muslim groups seeking to turf the powerful Shiites from their position of influence. A new civil war in Lebanon is not out of the question.

Even's Jordan's King Abdullah, struggling to contain the refugee-based Muslim Brotherhood movement, could face a widespread uprising if the Syrian civil war goes on much longer.

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Canada's mission will be incomplete and Ottawa would have to decide whether to extend it.