Why doesn't the US intervene to defend Saudi Arabia against the Houthis?

The US cautioned against a rapidly escalating situation in Yemen after Houthi rebels carried out a missile attack on the Saudi capital. In recent weeks, the Iran backed militants have also seized large portions of Yemeni territory following clashes with Saudi-backed government forces. The US is a very close ally to Saudi Arabia. Yet, is visibly unwilling to intervene on its behalf – why? Details and analysis by H. A. Hellyer, senior fellow in geopolitics at the Royal United Institute in Cairo.