Public opinion is a fearsome force, particularly when it moves decisively and suddenly.

It tends to trample objections, as independent-minded Americans discovered following the events of Sept. 11.

And at the moment, on the strength of one remarkably powerful image from Turkey, Canadian public opinion decisively and suddenly wants the prime minister to order his officials, right now, to start accepting Syrian refugees by the thousands, or, even better, by the tens of thousands.

The Liberal and NDP leaders, recognizing a potential election game-changer when they see it, have fully embraced the idea. Conservative Leader Stephen Harper has not.

Advocates of refugee admission put that down to his hawkishness, or a lack of pity. And Harper is partly to blame for that, given the awkward Conservative arguments that airstrikes by Canadian fighter jets over Syria and Iraq will in the long run help ordinary Syrians more than humanitarian aid.

But Harper is also the prime minister. And, as Ariel Sharon said after becoming Israel's leader, where you stand depends on where you sit.

Unlike Justin Trudeau and Tom Mulcair, Harper is regularly briefed by expert, senior government officials — public servants obligated by national self-interest, not polls, or the emotion that naturally arises from a picture of a small corpse face down on a beach.

They are no doubt telling him the obvious: that good public policy is not written in response to a single picture.

How much screening — and how quickly?

They would be pointing out that sensible refugee policy must consider the masses violently displaced from a number of countries other than Syria: Afghanistan, Iraq, Sudan, Libya, Eritrea, Iran and Somalia, to name just the seven other nationalities most commonly given formal asylum in Europe.

And they would also be telling their prime minister something that politicians and journalists have trouble saying in a liberal democracy whose sympathetic citizens are demanding action: the Syrian refugees are emanating from the world's current epicentre of ethno-religious violence, and they require screening.

Proper screening takes time.

Yes, most are simply wretched victims of evil men. But many are also tribally or religiously tied to the vicious regime in Damascus, or the country's equally nasty array of rebel groups.

Even worse, some may carry a hatred for the foreign powers carrying out the airstrikes that kill both fighters and civilians. Like Canada.

And there will always be a fraction of Middle Eastern migrants unwilling to adapt to pluralist Western values. European governments have coped with that.

As a Canadian official who has for decades inhabited the secret world told me this week: "I'd advise the prime minister to go as slowly as possible."

Answering the bells

All that said, the Canadian Security Intelligence Service doesn't make policy. It will carry out as many security checks as instructed.

The question is, how quickly and thoroughly does the prime minister want those checks done?

Conservative Leader Stephen Harper has rejected the idea his government hasn't acted quickly enough to the refugee crisis, while at the same time promising to increase the number of refugees Canada will accept. (Adrian Wyld/Canadian Press)

There is the simplest sort of check: does the applicant have a criminal record, or ring a bell in any of the country's security databases? That sort of check can be done quickly.

But what if a bell rings? Then allied services have to be consulted, and the reason for the bell-ringing investigated.

Is the individual the problem, or does he or she merely share the same name as someone with a darker past? If it is a matter of association, how serious was that association? Did they just sit beside one another on a train, or in a mosque, or were they together?

Government ministers, of course, prefer complete assurance, which of course never exists. You don't really know you have a problem until you have a problem.

A matter of political will

Ask the agents suddenly tasked with investigating the Air India bombing in 1985; Canada's security agencies, faced with the worst militant attack in Canadian history, knew precious little about Sikh radicals in British Columbia. They barely had any translators who spoke the language.

Ordering expedited screening and acceptance of migrants from a place like Syria is therefore a matter of political will.

In the United States, clearly, no such will exists. Clearances there take an average of two years, and the Americans have staggering resources, compared with Canada.

Having gone through Sept. 11 and two subsequent wars, Americans are utterly uninterested in admitting huddled masses of Middle Eastern Arabs. (And probably not too keen on Canada doing so, given the shared border.)

The paradox, of course, is that the United States, which invaded Iraq based on a lie, is directly responsible for what is happening now in the region. And so, to an extent, is Canada nowadays, given Harper's taste for militarism.

Some act of remediation toward innocents would seem morally imperative.

But that doesn't alter reality.

A lot is being made now of Canada's admirable decision in 1979 to admit so many Vietnamese boat people. Vietnam, though, was not Syria.

And the decision Harper — or whoever emerges as prime minister after Oct. 19 — must make about admitting Syrian refugees isn't just about compassion, or even morality.