It is usual for horror to be followed by hysteria. The unusual thing about the Paris massacre on Nov. 13 is that the most hysterical reactions have been thousands of miles from the scene of carnage.
American politicians appear to be suffering from post-traumatic stress disorder. Republican presidential candidate Ben Carson compared refugees to dogs. Rival Donald Trump vaguely threatened to do "things . . . that we never thought would happen in this country."
Yet mental disturbance is sometimes more dangerous when it is repressed. "The terrible events in Paris" were a "setback," declared a haggard and at times wild-eyed President Obama in a press conference that was painful to watch. Secretary of State John Kerry suggested that January's mass murder of staff at the magazine Charlie Hebdo had, if not "legitimacy," then at least a "rationale" because the magazine had made people "really angry."
Let's come off the prescription meds. The world faces three distinct threats: an epidemic of jihadist violence, most of it in the Middle East, North Africa, and South Asia; uncontrolled mass migration from these places to Europe; and the emergence of a "fifth column" of Islamic extremists within nearly all Western societies, including the United States. We must take care to distinguish each component of this terrifying trifecta.
The jihadist epidemic is by no means confined to Syria. Last year alone, according to the Institute for Economics and Peace, 32,658 people were killed by terrorism, compared to 18,111 in 2013. The two most deadly terrorist groups were Boko Haram and the Islamic State, which together were responsible for half of the fatalities. Nearly four in five attacks occurred in just five countries: Afghanistan, Iraq, Nigeria, Pakistan, and Syria. But the plague of jihad extends as far as the Malian capital Bamako, where Islamist gunmen slaughtered hotel guests on Friday.
There is clearly an urgent need to end the civil war in Syria. But let's not kid ourselves. Even if President Obama recalled David Petraeus and Stanley McChrystal to run a counterinsurgency campaign against the Islamic State similar to the one they ran against Al Qaeda in Iraq, the jihadist epidemic would still infect a dozen other countries.
Threat number two is a wave of mass migration to Europe that has been triggered by the Syrian crisis, but is by no means exclusively Syrian or even Middle Eastern. Statistics on the "country of origin" of asylum seekers in Germany show that they come not only from Syria but also from Albania, Afghanistan, Iraq, Serbia, and Eritrea.
At present, continental Europe has almost no way of controlling this influx, which grows larger with every passing month. Although the German government has now restored the Dublin regulation — which stipulates that asylum seekers can claim asylum only in the member state in which they entered the EU — in practice the entire apparatus for assessing applications has collapsed, as has the hotly contested scheme to redistribute asylum seekers between countries.
Even if every single one of the newcomers was an angel in human form, this would be a disastrous state of affairs, not least because continental labor markets are notoriously bad at integrating foreign-born workers. And no one should underestimate the domestic political backlash that is going to result.
The third threat is posed by a fifth column within Western societies of young Muslims or converts to Islam who join or at least sympathize with groups like the Islamic State. The overwhelming majority of these people are not refugees from Syria or anywhere else. Many are the children or grandchildren of an earlier wave of economic immigrants from former colonies. The biographies of the Paris terrorists tell the story.
What links the three threats together is the fact that at least six of the Paris terrorists spent time in Syria; and at least two of them were able to use the refugee route through Greece to return to France undetected. But that does not mean that the Syrian war or the immigration crisis were necessary for the Paris attacks to happen. Young Muslims are getting radicalized all over the Western world without going anywhere near Syria. Americans who think they can make themselves safer by excluding refugees are missing the point. The Tsarnaev brothers, who were responsible for the Boston Marathon bombings, were little different from the Abdeslam brothers, who helped carry out the Paris attacks.
The ancient Greeks believed that the gates of Hades were guarded by a monstrous three-headed dog. Like Cerberus, the monster we confront today has three heads: rampant jihadism, uncontrolled mass migration, and homegrown extremists. To defeat it, we shall need to keep our own heads very clear indeed.
Niall Ferguson is professor of history at Harvard University, a senior fellow of the Hoover Institution, Stanford, and the author of "Kissinger, 1923-1968: The Idealist.''