From "Proving Ground: Iran’s Operational Strategy in Syria" by Nicholas Hargreaves-Heald (http://smallwarsjournal.com/jrnl/art...rategy-syria):

Following the U.S. withdrawal from Iraq, Iran began to provide extensive aid to the Houthis, a group of Zaidi Shi’ites from the mountains of Yemen who were engaged in a civil war with the Yemeni government and the nations of the Gulf Cooperation Council (GCC). By 2016, the Houthis had seized a significant amount of Yemeni territory, including ground bordering Saudi Arabia. Iranian support, which was intended to develop the Houthis into a massive security threat to Saudi Arabia, followed the classic Quds Force TAA formula. Iranian training (which continues today[30]) has reached thousands of Houthi rebels[31], including Houthi women who have been mobilized to fight invading GCC forces.[32] Interestingly, this training has largely been provided by Iranian proxy organizations, such as Lebanese Hezbollah[33] and Liwa Fatemiyoun, a group of Afghan Hazaras recruited from Afghan refugee camps in Iran. [34] Iranian military aid to the Houthis has included sniper rifles, RPGs, anti-tank guided missiles (ATGMs), and various small arms, in addition to more sophisticated weapons technology. American officials have assessed that Iran has provided the Houthis with short-range, Iranian made Qiam and Shahab-2 ballistic missiles to attack Saudi Arabia, Iran’s principal adversary in the Arabian peninsula.[35] The Houthis have launched dozens of these missiles (and others based on their design) at Saudi targets such as the al-Yamama Palace (December 2017)[36], the city of Najran (March 2018)[37], the capital city of Riyadh (March 2018)[38], and the Jizan Regional Airport (April 2018).[39] Authorities have also assessed that Quds Force provided the technology for the Houthi’s explosive, remote-controlled “drone boat”, which they used to attack a Saudi warship on January 30, 2017[40], and the use of an explosive-laden Qasef-1 “suicide” drone to attack a Saudi Patriot batteries in February, 2017.[41] On top of this, Quds Force has been reported to advise Houthi leaders off the battlefield, and encourage them to launch kinetic operations against GCC targets. [42]

There are, however, some indications that Iranian operations within Yemen are not limited to Quds Force’s standard TAA approach. Over the course of the last year, over forty IRGC (it is unclear whether these are Quds Force or IRGC-GF operatives) and dozens of Hezbollah casualties have been reported in Yemen.[43] While many of these casualties have been reported as the result of GCC airstrikes against Houthi targets far behind the front lines, almost a dozen have been reported to take place within a mile of active combat zones and front lines. Such casualties would suggest that Iranian forces are either actively participating in combat in Yemen or advising Houthi rebels close to the battlefield.[44] So far, however, there has only been limited evidence to support this hypothesis.[45]