In a complicated turn of events, and despite being part of the same coalition fighting Houthi rebels, forces loyal to the United Arab Emirates (UAE) and those loyal toSaudi Arabia appear to have turned on each other.
Fighting has been ongoing for days, killing dozens, as a battle for control of the southern city of Aden intensifies.
The UAE-backed Southern Transitional Council wants the Yemeni government out.
So what does it all mean? And how might it affect the war?
