It didn’t take long for border security to claim the top spot among issues emerging from the presidential conventions. Every day news flashes describe the latest atrocity somewhere in the world. An immigrant, usually a demented Muslim, murders his native hosts. The dilemma of how to balance compassion for humanity with the necessity of security becomes more painful with each bloody attack. Civilization has no choice but to protect itself from barbarians.

In an innocent time long ago, the poet Robert Frost observed that “there is something that doesn’t love a wall.” That was then. Now walls mean not separation but safety, which is something everyone does love.

Americans witnessed images and sounds from a relentless barrage of horrific terrorist attacks abroad. The likelihood of an American being selected for death by radical Islamic terrorists in a U.S. population of more than 300 million is remote. But who can be satisfied to whisper, “I’m glad it wasn’t me,” after each atrocity? The barbarism gnawing at the foundations of civilization endangers everyone.

Mr. Trump’s victory over his Republican opponents can be attributed in part to his early recognition that immigration policy is national security policy. Voters have understood this for years. Democrats risk all by refusing to accept that their failures at securing the border are in fact failures. A similarly stubborn vow has put German Chancellor Angela Merkel teetering on the edge of collapse.

Terrorism was listed as an “extremely important” issue for 92 percent of Republicans and 82 percent of Democrats polled earlier this year by Gallup. Donald Trump has made national security his signature issue. Hillary Clinton is betting everything that voters are as blind as she is.