Thousands of Saudi troops have been deployed along these desiccated hills, struggling to halt cross-border attacks by Yemeni rebels who fire rockets and carry out lethal ground incursions.

The Yemeni fighters have killed and captured hundreds of Saudi soldiers in a conflict that presents Saudi Arabia with the biggest challenge to its territory in years. Thousands of mortars and crude rockets have slammed into schools, mosques and homes in Najran, a city of several hundred thousand people only a few miles from the mountains of northern Yemen.

The border assaults have come in response to a devastating air and ground war that a Saudi-led military coalition launched in Yemen last year. That conflict has in turn spilled across the border with devastating consequences, said Maj. Gen. Saad Olyan, who commands more than 20,000 Saudi forces along the 1,100-mile southern border. […]