The advance of Yemen's Houthi movement into Sanaa in September 2014 and its subsequent actions are the culmination of a Houthi expansionism that has been growing since 2011. But the movement and its conflict with Yemen's central government go back much further.

The Zaydi (a Shia sect) Houthi movement (also Ansar Allah or Shabaab al-Mumanin), is a revivalist movement that emerged in the northern governorate of Saada in the 1990s, in response to growing Salafi influence in north Yemen. It was led by a Zaydi sayyid (descendant of the Prophet Muhammad) and member of parliament, Hussein Badr al-Din al-Houthi.

Fifty-five per cent of the Yemeni population is Shia, the vast majority of whom are Zaydi, largely concentrated in the north and west, including the capital, Sanaa. A Zaydi Imamate ruled the country until 1962, when it was overthrown in a coup with Egyptian backing, sparking a civil war in which Saudi Arabia supported the Imam's forces. Zaydism holds that the only legitimate Islamic government is rule by an imam, who must be a sayyid. In many practical respects, however, it is hardly distinguishable from the Shafi school of Sunni Islam, the predominant form practiced in Yemen.

The Houthi movement started as the 'Believing Youth' (Shabaab al-Mumanin), a summer educational programme to promote Zaydi beliefs and culture. This effort may have had the tacit support of the then government as a means of balancing rising Salafi influence in the north of Yemen. Such influences had grown with the return of Yemeni mujahideen from Afghanistan. However, in a wider strategy of divide and rule (President Saleh once compared governing Yemen to "dancing on the heads of snakes"), it is likely that Salafi groups may have also had some government support. Around the same time, the regime encouraged the creation of al-Islah, an Islamist party that, at least in its early years, was at different times in coalition government or quiescent opposition.

The start of the War on Terror, which the Yemeni government officially supported, led to rising tensions between the Houthi movement and the regime. The movement's slogan, "God is Great; Death to America; Death to Israel; Damnation to the Jews; Victory to Islam", was seen as a criticism of the regime, which was supporting American operations against al-Qaeda, especially when the slogans were  shouted in President Ali Abdullah Saleh's presence in Sanaa's Grand Mosque. This led to an effort to arrest al-Houthi, prompting the first Saada war and culminating in al-Houthi's death in September 2004, with the operation leading to the deaths of over 1,000 people.

On al-Houthi's death, his father, Badr al-Din al-Houthi, took over leadership of the movement. On his death, he was in turn replaced by al-Houthi's brother. The Houthi movement went through five further rounds of conflict with the Yemeni government between 2005 and 2010, with the 2009 fighting season leading to a Saudi intervention after Houthi forces crossed into Saudi territory, killing three soldiers. These successive wars led to the internal displacement of approximately 100,000 Yemenis from the northern governorates, and access for aid agencies and the media was severely constrained.

The Arab Spring and ensuing political instability in Yemen undermined the regime's fight against the Houthi movement. Following the departure of President Saleh under a controversial Gulf Cooperation Council (GCC) plan that gave him amnesty for past actions and the right to return to Yemen, a 'National Dialogue Conference' (NDC) was instituted to propose reforms to Yemen's political system. The Houthi movement played no part in the GCC agreement, but did participate in the NDC, bolstering its political appeal beyond its traditional base, and, in the absence of effective government to prevent it, the movement reinforced its hold over the northern governorates.

However, the Houthis opposed the NDC's main recommendation, that Yemen should be divided into six federal regions, fearing that it would diminish their territorial powerbase. According to an International Crisis Group alert in September, the Houthis were acting to "become dominant in the north and more powerful on the national level. Even as [its] leaders make demands of the central government in Sanaa, their militias are fighting al-Islah-affiliated tribesmen and military units in Jawf governorate in what is widely viewed as an effort to create facts on the ground to force a renegotiation of the six-region federal structure proposed by the NDC".

Through the summer of 2014, the movement advanced to the south, capturing the city of Amran in July. By early autumn, Houthi groups set up armed camps around the capital, conducting pro-Houthi demonstrations throughout the city, and in September they occupied it, prompting the resignation of the transitional government. The ensuing peace deals signed with President Hadi recognised his authority, but required the appointment of Houthi advisers to his government; however, further tensions have led to a growing war of words between Hadi and the Houthis, and accusations on the government's part that the Houthis are plotting a full blown coup.

Since the start of its insurgency, the Houthi movement's Shia origins have raised suspicions ( denied by the group) that it is backed financially and militarily by Iran. Despite the Houthis coming from a different Shia sect from that dominant in Iran, such accusations feed increasingly divisive and sectarian rhetoric, accompanied in the case of Salafi-jihadi groups such as al-Qaeda in the Arabian Peninsula (AQAP) by violence. There have long been accusations that the Houthi conflict in Yemen is part of a wider regional struggle between Saudi Arabia (which is known to support Salafi groups in Yemen) and Iran.

Ultimately, however, the Houthi movement's aims seem to be focused on Yemen, making common cause with the separatist Southern Movement. Their rhetoric has been focused on government reforms; in a recent speech, the group's leader accused President Hadi of being "at the forefront of the forces of corruption". Their detractors fear that they wish to revive the Zaydi Imamate, fears bolstered by the fact that their leaders are sayyids. But whatever their motivations, the movement's past actions and apparently growing territorial ambitions indicate a desire, common to many groups in Yemen, for substantial autonomy from – or control of – the central government.

This Backgrounder was published on 28 December 2014.

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