The Saudis and the Houthis began fighting in 2015 when the kingdom entered Yemen's civil war on behalf of the internationally recognized government.The Houthis are Shia Islamist political and paramilitary organization associated with Iran. Saudi Arabia is a Sunni Muslim state. 

The latest attack on the international airport in Saudi Arabia's capital of Riyadh killed 12 people and injured over 300 others, civil aviation authorities said early Sunday, with some in critical condition. A Saudi-led coalition blamed Iran-backed Houthi rebels in neighboring Yemen. There has been a surge in attacks initiated by Houthis, including against other Saudi airports, military facilities and oil infrastructure after Saudis launched a major military offensive against them.

Until recently, the Houthis had been largely on the sidelines of the Iran war. Now the rebels are a way for Tehran to apply pressure on a key U.S. ally.

President Donald Trump said he was evaluating whether the US should join Saudi strikes against the rebels. "We may. We're going to look at it," he said. "We just found out about the recent attack. So we'll make a decision. We move very quickly." When asked if the US might get involved, Trump responded, "We'll find out."