Belgium attacks said to be 'unlikely to be duplicated in the United States.' But how do we keep it that way?

Matt Zapotosky & Adam Goldman (The Washington Post) write:

The apparently coordinated bombings that killed more than 30 people in Belgium are unlikely to be duplicated in the United States, which is separated by an ocean from Islamist extremists fighting in Syria and Iraq and has seen far fewer of its people traveling there, former intelligence and counterterrorism officials said.

I certainly hope they are right. But if so, how do we keep it that way? If it's just the ocean, that's wonderful; it's not going away. But I suspect that these days oceans don't matter as much as they once did.

What happened in Belgium that we should be avoiding? I have no preconceived answer to that question. But I'd certainly love to know what that answer is.