US won't defend Saudi Arabia? Houthi missile hits Riyadh

The US has warned of a quickly growing crisis in Yemen after Houthi rebels fired a missile at Saudi Arabia's capital. In recent weeks, the Iran-backed militants have also seized large areas of Yemeni territory in fighting with Saudi-backed government forces. The US is a close ally of Saudi Arabia. Yet it is clearly unwilling to intervene on Saudi Arabia's behalf. Why? Analysis by H. A. Hellyer, senior fellow in geopolitics at the Royal United Institute in Cairo.