Saudi Arabia bombed Yemen's important port city Mukalla on Tuesday. The strike came after a weapons shipment from the UAE arrived for separatist forces in Yemen. Riyadh warned it views Emirati actions as "extremely dangerous." The bombing followed rising tensions around the advance of UAE-backed separatists from the Southern Transitional Council. The council and its allies supported the UAE's presence. Some others demanded that Emirati forces withdraw within 24 hours. Saudi Arabia issued a strong ultimatum, telling the UAE "Don't cross our red line!" The UAE called for "restraint and wisdom" and denied Riyadh's allegations. Soon after, the UAE said it would pull back its remaining troops in Yemen. It is unclear if the separatists will give up the territory they recently captured. This confrontation threatens to open a new front in Yemen's decade-long war. Forces allied against the Iranian-backed Houthi rebels may now fight each other. The clash worsens already strained ties between Saudi Arabia and the UAE, neighbors who compete on economic and regional political fronts, especially in the Red Sea area. Tuesday’s airstrikes and Saudi Arabia’s ultimatum represent the most serious clash between the two Gulf powers in decades. The strike follows earlier Saudi air attacks on the Southern Transitional Council on Friday, which analysts saw as a warning to stop their advance.