Cheap shots should be ignored in politics. So forget New Democratic Leader Thomas Mulcair's jibe that Canada is fighting "Harper's war" in joining the military coalition against the Islamic State.

This is not Stephen Harper's war. Whether or not you agree with the bombing countries' strategy and tactics, this is a multinational effort to contain and eventually defeat a ghastly group. Reading between the lines of what the Prime Minister said, he had a hard decision.

Once he decided to join the coalition, he had to participate militarily, but at a minimum cost, which is what six CF-18s, humanitarian assistance and trainers represent. Anything more might have inflamed public opinion at home; anything less might have offended Washington and other allies. Put another way, once Mr. Harper decided to participate, he had to walk a narrow line.

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It isn't easy to rebuff your best friend and protector, the United States. It's more difficult still to do so when your other best friends are in the fight – countries such as Britain, France, Australia and the Netherlands. They would scorn the kind of explanation advanced by Liberal Leader Justin Trudeau – that a country can be engaged militarily through humanitarian aid.

A considerable time before Canadian troops left Afghanistan, Mr. Harper had decided the fighting mission there had been oversold and had stretched on too long. Tellingly, he set a date for Canada's exit (except for training), informed other countries and Afghan authorities, and the troops duly came home on that date.

The Islamic State campaign will be very long, however it unfolds. It was significant, therefore, that Mr. Harper set a six-month time limit on Canadian engagement, at which point the engagement will be reviewed.

True, it will be difficult to pull out once Canada is engaged, but the fact that the Prime Minister set out a time frame indicates his awareness of the missions' multiple difficulties. He does not want another Afghanistan. Whether he can avoid one remains unclear.

He also has his eye on domestic public opinion, as prime ministers always do. Six months means that if the mission is desperately unpopular at home, or takes a military turn for the worse, he would have the option of pulling Canada out after six months – which is about six months before the next scheduled election.

Mr. Harper has complained, both publicly and in private to other world leaders, about the difficulty of sorting friends from enemies in Syria. So it was significant that he said Canadian forces will only be involved in Iraq. They will not – at least for now – be engaged in the Syrian cauldron.

Democratic governments that enter wars often oversell what participation means, because they feel that is the way to win public support. The Harper government has hyped the threat to Canada from the Islamic State. The trouble with overselling is that it makes getting out harder. It would have been more intellectually honest, but more politically difficult, to explain some hard truths to Canadians.

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It could have started with the limitations of air power, which is necessary but not sufficient to prevail in a long conflict. Islamic State fighters have already done what would have been expected, facing Western air power: They've hidden military targets, reduced communications and split into smaller groups.

Air power can inflict some damage and restrict the militants' mobility, but it will not drive the Islamic State from territory it already occupies, especially large cities such as Mosul and Aleppo. And air power always inflicts collateral damage on civilians, which can turn local populations against the bombing powers.

Ground forces will eventually be required in Iraq and Syria, but at the moment, there are none capable of winning major battles. At best, the forces in Iraq will be an unwieldy combination of Kurdish fighters, Shia militias and the Iraqi army, since no Western power or Arab state will send tens of thousands of soldiers and massive quantities of equipment into this fight.

In Syria, the forces will be so-called "moderates" and the forces of President Bashar al-Assad, whom the United States and its allies would still prefer to overthrow. As has often been noted, the enemy of your enemy can become your friend – at least temporarily.