Watch: Who are the Houthis, and why are they so hard to fight?

The Houthi movement, backed by Iran in Yemen, launched missile attacks on Saudi Arabia, killing at least 14 and injuring over 300 at Riyadh airport. They also targeted oil facilities and Dammam airport in retaliation for what they call 'brutal Saudi aggression' in western Yemen. Saudi Arabia has vowed to respond to these attacks, while security expert Frank Gardner explains that the Houthis' precision strikes and guerrilla tactics make it difficult for Saudi forces to gain the upper hand.