There ain't much anybody can do. It's one of our insoluble problems.
For those of us outside areas of the Middle East and Africa where one or another radical movement established itself, the problem may grow with the defeat of the extremists in areas they had once controlled.
The worry comes not only from individuals that had served in Syria or Iraq and then go home to wreck havoc among the infidels.
Those can be identified and watched.
Even that is difficult Europeans have been killed by those who slipped through the cracks. And screening returnees from Syria or Iraq does not deal with the influence of extremist Islamic preachers and web-sites, as shown by the group of Moroccans who created the recent problem in Barcelona, and could have done something even greater if they didn't have a "work accident" with explosives while getting ready.
An even more elusive problem is with the tens of millions of Muslims, who came to Europe or the US as refugees from recent chaos, or migrants in decades past. The overwhelming majorities in both groups appear to be intent on living better than elsewhere. Their labor is required by stable or declining populations, and by natives who refuse dirty and low paid work.
There are individuals intent on revolution among Muslims refugees or migrants. However, the effective screening of thousands or millions moving at peak times stretches any government's capacity.
Violence may not come only refugees or migrants, but from their children and grandchildren. Individuals ignited by Islamic preachers, or by Islamic web sites come from families that have assimilated.
There's no turning back history. What to do with Muslims already established, with citizenship in Europe or the United States? The norms of western societies do not tolerate invasive policing. Freedom of expression and religion retard efforts to monitor sermons and schooling.
There are Muslims who express themselves against preaching that is anti-Christian or anti-Semitic, but there are also norms in Islam that support action against non-Muslims. Most Muslims are not inclined to speak out against what they hear from religious or political leaders. Individualism hasn't caught on among Muslims to the extent that it has among Christians and Jews..
Israel is better set than other countries to deal with the threat. The explanation begins with the dominant Jewish population, the history of warfare and terror, and resulting willingness to compromise issues of civil rights for the sake of security. Israelis are used to screening at the entrance to public buildings and parking lots. Ethnic profiling makes it easier on those who speak Hebrew without an Arabic accent.
It's tempting for Israelis to feel relief as well as sadness in response to terror attacks in Europe or the US. The hope, not quite fulfilled, is that the experience will increase the identification of others with Israel's problems, and soften criticism of Israel by governments and activists.
Israel's concern for security also comes up against domestic and international politics. A warning came from the government's reversal of its effort to screen Muslims going to pray on the Temple Mount. The anticipation of greater commotion caused authorities to back down from security measures similar to those used by Saudi Arabia at its holy sites.
Life was better in western societies before 9-11 and lesser incidents. It was quicker and easier to enter an airport and go from check-in to the plane.
Now planes are safer, as well as airports beyond the security checks.
That's been a manageable fix. Other obvious targets are more difficult to protect, such as the flood of people entering railroad or subway stations at rush hour, or a sport stadium..
Barcelona reminds us that a terror attack need not involve anything more sophisticated than a motor vehicle, driven at speed into a crowd.
It should be possible to recruit Muslims or other Arabic speakers to provide intelligence about what is said in Mosques, posted on web sites, or talked about among those on the list of extremists.
However, political sensitivities are part of the problem. American and European officials have dithered at length before deciding that deaths and injuries have come from Islamic terror. The term itself was a problem for Barack Obama. The plan of the Barcelona terrorists to implement a more grandiose scheme at a Cathedral raises the issue of how thorough Christian authorities will screen church-goers.
Terrorists crave crowds, and are willing--even desirous--of risking death.
Is there nothing other for Americans and Europeans than to accept casualties from Islamic extremists, more or less like they accept what has been the much greater carnage from road accidents?
There's no shortage of preaching to get along with others, reconcile disputes about historical narratives, and expand teaching the virtues of brotherly love. There's also the history of Christian extremists getting over the urge to kill non-Christians and Christian rivals. But there's no sign that the mass of Muslims are close to Christian accommodations with themselves and others.
Charolottesville illustrates tensions that go beyond Islamic extremism. Home-grown tensions between American Nazis, the KKK, Antifa, Black Lives Matter and their constituencies are enough to produce mayhem. Not helping are Donald Trump's convoluted efforts to explain himself and stay in touch with his constituency.
One needn't relax vigilance, or efforts to acquire intelligence and prevent incitement and attacks. Problems of politics and morality will require coping, however that is feasible.
It's also worthy to do what is possible to reduce traffic accidents.
In both fields, however, we should be aware of the limits.
There are unpleasant realities involved with automobiles and Islam, best viewed as irreducible to zero casualties.
It's tempting to idealize the past, but it wasn't free of violence, had greater risks of disease, and overall shorter life spans.