President Obama on Sunday promised to “redouble” U.S. efforts against Islamic State, which shows he isn’t deaf to the political impact of Friday’s murderous assault in Paris. But why should anyone believe him? After years of dismissing the rising terror threat, Mr. Obama needs an epiphany if he doesn’t want to be remembered as the President who allowed radical Islam to spread and prosper.

“It is an act of war that was waged by a terrorist army, a jihadist army, by Daesh [the Arab name for Islamic State], against France,” said...