The capital of Yemen, the Arab world’s poorest and perhaps most chronically unstable nation, has new masters. Anti-American Shiite rebels man checkpoints and roam the streets in pickups mounted with anti-aircraft guns. The fighters control almost all state buildings, from the airport and the central bank to the Defense Ministry.

Only a few police officers and soldiers are left on the streets. Rebel fighters have plastered the city with fliers proclaiming their slogan — “Death to America, death to Israel, a curse on the Jews and victory to Islam” — a variation of a popular Iranian slogan often chanted by Shiite militants in Iraq and supporters of Lebanon’s Hezbollah.

While the world has been focused on the fight against Islamic State militants in Syria and Iraq, Yemen — located at the southern tip of the Arabian Peninsula — has seen its own sudden, seismic upheaval when Shiite rebels known as the Houthis overran Sanaa two weeks ago. […]