SANAA, Yemen — On March 25, tribal gunmen loyal to Yemen's Houthis, along with Yemeni military forces, tightened their siege on the southern city of Aden, which President Abed Rabbo Mansour Hadi declared as a temporary capital after he fled there from Sanaa more than six weeks ago.

Two weeks earlier, Houthi gunmen, in conjunction with some Yemeni army units, conducted a military exercise in Saada province on Saudi Arabia's southern border. These Houthi activities may have been the direct cause of the largest-ever bombing campaign against Yemen, which has not fought a war with another country in 80 years. Ongoing internal wars, however, have torn Yemen apart — including the current one between Hadi's forces and the Houthis; al-Qaeda in the Arabian Peninsula (AQAP) and the army; and the six wars between 2004 and 2009 in Saada between the Houthis and the government. Yet, March 26 was a defining moment in Yemen’s modern history, as Sanaa residents woke up to the sounds of continuing airstrikes.

The Houthi threat against Saudi Arabia has increased. Saudi Arabia considers the Houthis an extension of its traditional foe, Iran. Yemen’s legitimate president became trapped in Aden after escaping from a monthlong house arrest imposed by the Houthis, who surrounded his house in Sanaa. He escaped Feb. 21, the third anniversary of the referendum that made him president. If Aden falls into the hands of Ansar Allah, the Houthis would effectively tighten their control over the country’s key locations, especially Aden's international airport and the Bab al-Mandeb strait. The fall of Aden is considered a threat to one of the largest oil routes in the world and would affect the global oil economy.

The Houthis' military progress from their northern stronghold has been an unprecedented source of concern for Riyadh since mid-2014. That concern culminated in September 2014, when the Houthis tightened their grip on Sanaa, whereby the army and security establishments came under their direct control.