Twenty-five years after Sept. 11, Americans will again remember the smoke over lower Manhattan, the collapsing towers, and the passengers who fought back aboard Flight 93. We will honor the dead and repeat the vow: Never forget.

But remembrance is not the same as understanding. We have remembered 9/11 as a terrorist atrocity, but we have failed to confront the ideological challenge it represented — one we remain reluctant to name, understand, and resist.