PARIS, France—The warning came not long before it was time to board the train.

The mass shooting in a Moscow concert hall that killed at least 137 people, for which the Islamic State claimed credit—such a twisted “achievement” over which to take a bow—prompted the heightened security.

The French had reason to feel wary. The massacre in Russia carried with it echoes of a similar horror in Paris in 2015, when a series of coordinated attacks left 130 people dead and another 400 wounded.

With the tourist season about to begin in earnest and the Paris Olympics approaching, France offers many potential targets for terrorists.

On the nearly three-hour ride from Avignon to the Gare de Lyon—Paris’ train station for travelers coming from and headed to the south—I watched the passengers for signs they were concerned.

And saw none.

Across the aisle, a little boy midway between the baby and toddler stages amused everyone within earshot with his burbles and exclamations. Taking the train wasn’t a trip for him. It was an adventure and he wanted everyone around to know it.

After he fell asleep, the travelers settled into their routines. Most scrolled on their phones. Others read or did work on their laptops. Still others dozed.

When we arrived in Paris, the interlude ended.

Security personnel in their distinctive black uniforms were positioned every 15 or 20 steps. They scanned the crowd for any hint of trouble or danger.

At the turnstiles where passengers enter and leave the boarding platform, guards pulled disembarking travelers aside to open and inspect their bags. Some passengers even got pat-down body searches.

That wasn’t the case when I left from this train station less than a week ago to amble around Provence. Then, anyone with the proper ticket could stroll onto his or her train undisturbed.

But that was before the Islamic State made clear it was issuing statements again—and punctuating them with tragedies on a large scale.

More caution seems merited.

The question is whether it will make a difference.

Russia is one of the most closely guarded police states on the planet. The Russian people must endure invasions of privacy and deprivations of personal autonomy most residents of the self-governing world would struggle to tolerate for an hour, much less their entire lives.

And yet the murderers associated with the Islamic State circumvented all the Russian security protocols and brought death by the dozens to a music hall in Moscow.

Such is the nature of warfare in this age.

It is not only undeclared but asymmetric and seemingly random in nature. The ones who practice it seek not to destroy or capture traditional military targets. Instead, they want to use fear as a weapon of mass destruction by creating the impression that anyone anywhere at any time can be subject to a deadly attack.

Hence, the name terrorists.

They seek to shake our sense that the world operates in a rational, predictable way.

And they draw strength from counterproductive responses to their surprise murder sprees and other evil acts. When they see how desperate their adversaries are to draw them into more traditional kinds of war, they know they are gaining ground.

Consider the situation in Gaza.

What Hamas did—the murders, the rapes, the kidnappings—was evil in all ways, but the Netanyahu government’s response on behalf of Israel likely only will encourage similar attacks in the future.

Benjamin Netanyahu’s overwhelming use of force has left civilians—not the hostage-takers, rapists or murderers of Hamas—feeling the bulk of Israeli rage. Many of those in Gaza who have lost loved ones will want to strike back.

That is the ultimate goal of terrorists.

They seek to make us think and act like them.

And at times, they seem to be winning.

Once I cleared the turnstiles, I did a quick scan of the throng to see if anything seemed amiss. My eye caught nothing worthy of concern.

I did, however, see the little boy who amused everyone in the train car.

His mother carried him in her arms.

He was still sleepy from his nap, so he rested his head on her shoulder.

Nestled in his mother’s arms, he’s innocent—in a world where innocence itself seems so often imperiled.