Ostensibly allies in the Yemen conflict, Saudi Arabia and the UAE are backing different, frequently opposed, forces. The UAE’s priority is countering Islamist groups, leading it to back southern separatists who oppose the Saudi-backed government. Further chaos beckons.

After more than two years of incessant conflict, Yemen is once again proving the place where swift military operations in the Arab world go to die. Saudi Crown Prince Muhammad bin Salman’s hopes for a decisive...