Yemen:
• Yemen has never been a stable country, even prior to the Arab Spring.
• Currently the war is between the Houthis (supported by Iran) in the north against Hadi-loyalists (supported by Saudi-Arabia and co.) in the south and ISIL/AQAP in the east.
• The Arab Spring allowed Houthi Shias in the north of Yemen to mobilize just like the rest of the region.
• Al-Qaeda in the Arabian Peninsula (AQAP) has always been active in Yemen. The attack on the USS Cole was by the Yemeni based Al-Qaeda. [Source]