Why Yemen's Houthi rebels have only grown stronger through years of war

Now, as Iran-backed Houthis advance on a key maritime chokepoint and strike Saudi oil facilities, rattling world markets, there is little that can be done to stop them that hasn't been tried before

At various times over the last decade, Saudi Arabia, the United Arab Emirates, the United States, Britain and Israel have carried out punishing air raids on Yemen's Houthi rebels -- only to see them grow even stronger.

Now, as the Iran-backed Houthis advance on a key maritime chokepoint and strike Saudi oil facilities, rattling world markets, there is little that can be done to stop them that hasn't been tried before.

That's because the Houthis operate in a large and mountainous territory where they can disperse fighters and military infrastructure while threatening major oil infrastructure and key trade routes. Iran and its allies in Lebanon and Iraq provide expertise, training and sophisticated weapons, according to UN and other experts, and the Houthis' hard-line ideology does the rest.

"They are the most committed, the most ideologically pure and the most aggressive" of Iran's regional allies, says Michael Knights, who has written extensively about the Houthis and is now head of research at Horizon Engage, a New York-based strategic advisory.

"They've been developed by the Iranians in a fast-track way. Things that took the Lebanese Hezbollah 20 years to develop, the Houthis have developed in five."