Obama in the Oval Office on Sunday night. (Credit: Doug Mills/The New York Times)

President Obama sought on Sunday to calm a jittery American public after the terrorist attack last week in California, delivering a prime-time address designed to highlight the government’s campaign against an evolving threat.

Speaking from behind a lectern in the Oval Office, Mr. Obama bluntly acknowledged the heightened fears that followed attacks in Paris and in San Bernardino, Calif., which his administratio