Saudi Arabia and the United Arab Emirates have pumped billions of dollars into fighting Houthi rebels in Yemen, but the Gulf states' three-year campaign risks being derailed after their local allies turned on each other this week.
Riyadh and its allies see victory in Yemen, where they are backed by U.S. weapons and intelligence, as vital if they are to counter Iran's growing influence in the Middle East, a priority for Saudi Crown Prince Mohammed bin Salman.
But coalition prospects have been dimmed by an armed uprising this week by fighters in southern Yemen, who have been backed the United Arab Emirates (UAE), against government forces until now on the same side.
This comes at a time when the coalition war effort has already been running into trouble. Late last year, the coalition moved quickly to support former president Ali Abdullah Saleh when he seemed to be about to end his backing for the Houthis, but he was killed by the Houthis.