Allies of the former Hadi government: President Abed Rabbo Mansour Hadi came to power in November 2011 after Ali Abdullah Saleh stepped down because of increased opposition to his autocratic rule during the height of the Arab Spring. Hadi was backed by the United States and other Western nations, especially in the fight against al-Qaeda in the Arabian Peninsula (AQAP), the global terror network's powerful Yemeni branch, which is based in the south. Sunni Saudi Arabia had funneled billions of dollars to the now-defunct government.
Houthi allies: The Houthis, a Shiite rebel group based in northern Yemen, increased attacks against the Hadi government and eventually were able to drive the president out of Sanaa, the capital. The Houthis are allegedly backed by Iran, a Shiite nation. Saudi Arabia and Iran have long been competing for power in the Middle East, and the Houthis' ascendancy in Sanaa is seen as a big strategic win for Iran and a huge loss for the Saudis.
Rival groups: AQAP has been in the news lately because of the group's claim of masterminding the Charlie Hebdo attacks in Paris in January. The group has been fighting against both the Houthis and the Hadi government. Also active in the south is a separatist movement made up of Sunni rebel groups that has at times been linked to AQAP, though the connection remains unconfirmed.