The moniker “over the horizon” has been mouthed by defense officials from the new commander in chief down, but exactly how the United States will confront violent extremist forces in Afghanistan after a full withdrawal remains a mystery.

Former President Donald Trump’s first Air Force secretary, Heather Wilson, says she is not sure how the Defense Department will accomplish the coming mission, which senior military officials acknowledge will be challenging. And top Pentagon officials hand-picked by President Joe Biden can’t yet say either.