The Houthis have made rapid advances in Yemen, sparking a dormant civil war back to life in the Middle East, threatening their neighbour Saudi Arabia. We speak to the Iranian-American academic Vali Nasr about how the Islamist militia has become the key ally for Iran in its war with the US and Israel, and why the advances the group has made in Yemen represent a 'major strategic defeat' for Riyadh. To learn more about why the Houthis success has global implications, watch our latest FourSight now. Picture: JACOVIDES-POOL/SIPA/Shutterstock #middleeast #iran #houthi #yemen #saudiarabia #usiranwar