After four months of setbacks, fighters backed by Saudi Arabia have seized the offensive in Yemen’s war, taking control of a major city and pressing to expand ground operations against rebel forces.

The shift in momentum after the Saudi-led coalition failed to make headway appears to be due to the arrival since mid-July of hundreds of Yemeni fighters who had been secretly trained in Saudi Arabia. The contingent could help turn the tide in the war, which pits Shiite Houthi rebels from the north against largely Sunni forces aligned with exiled President Abed Rabbo Mansour Hadi.

This week, Saudi-backed fighters attacked areas north of the southern port city of Aden in an apparent effort to enlarge recent gains there against the Houthis. Residents said the fighters are preparing to assault the rebel-held al-Anad base, Yemen’s biggest military airfield, 30 miles north of Aden. […]