When a bomb exploded on a London Underground train yesterday morning, Donald Trump was quick to put the incident to his own political use.

“Another attack in London by a loser terrorist,” he said on Twitter. He continued: “Must be proactive! The travel ban into the United States should be far larger, tougher and more specific — but stupidly, that would not be politically correct!”

Trump’s assertion, mirroring similar ones in the past, was that the attack demonstrated the need to curtail new immigrants to the United States. Never mind that, when Trump tweeted, it wasn’t clear who had planted the bomb — a missing bit of information that British authorities pointedly noted in response to Trump. Never mind, too, that recent attacks in the United States by individuals inspired by radical ideology were committed by American citizens.

And never mind that there is a significant and often unmentioned protection the United States enjoys, which Europe does not: the Atlantic Ocean.

Dan Byman is a professor at Georgetown University and senior fellow at Brookings who served as a staff member on the 9/11 Commission. He spoke by phone with The Washington Post to explain how the geography of the United States offers a distinct advantage.

That advantage isn’t only about the relative difficulty of getting to the United States from the Middle East, as compared with getting to continental Europe, but that’s a lot of it.

“Europe has all of these land entry points,” Byman said. “And especially when you’re talking about the Islamic State — notionally, you could drive from Syria to Paris.”

To get to the United States is much harder. To gain access directly, you have to arrive by boat or air, limiting the number of entry points that the Government needs to police and limiting the number of people who can attempt it. Travel over such a distance is also more expensive, leaving more of a paper trail.

To gain access indirectly — travelling to another country on the continent and making your way into the United States — may alleviate some of the scrutiny of the authorities, but introduces a new complexity.

“From a terrorist’s point of view, they have to worry about the odds of success,” Byman said. “Many groups can’t afford to try 20 times and say, ‘We’ll lose some but it will be OK.’ They have to worry that one blown operation will have profound consequences.”

Sure, he pointed out, lots of things get smuggled into the United States with regularity. But those are generally funnelled through criminal operations, and a criminal operation that was considering helping to smuggle a terrorist into the country would certainly recognise that the repercussions of being caught doing so would be significant. A terrorist organisation considering working with a criminal group to be smuggled into the country, for their part, would likely recognise that the criminals may just as readily turn them over to the authorities.

“All of these are about probabilities,” Byman said. “This doesn’t stop anything; it just makes it harder.”

He also noted another way in which America’s isolated geography affected the likelihood of terrorist infiltration.

“There are historic reasons European states have significant Muslim communities,” he noted.

Western colonialism in Africa and the Middle East, for example, helped to build communities in Europe with roots in those nations. America’s colonial extensions were centred elsewhere, like Latin America.

Those existing communities meant that when people sought to emigrate from the Middle East more recently, they tended to focus on nearby countries with populations that shared a culture and language. If you were looking to emigrate from Algeria, would you go to Miami or would you travel to Paris?

The density of those local populations in Europe also makes it easier for those looking to blend in to do so.

“The number of people who are skilled enough to operate in an environment where there aren’t many people who speak their language and whose culture they don’t know? It’s a lot harder,” Byman said.

This goes back to Byman’s other point: when it is harder to sneak terrorists into a country and there are fewer terrorists who could be successful once they’re there, that sharply decreases the number of people who would be successful.

This isn’t just theoretical.

“We know from some of the records we’ve seen over the years from groups such as al-Qaeda that they see the United States as a harder place to get into than they do Europe,” Byman said.

• Philip Bump is a correspondent for The Washington Post based in New York City