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Just as the Protestant tradition is subdivided into Methodists, Presbyterians, Congregationalists and others, Shiite Islam is also subdivided. Houthis belong to the Zaydi branch.

From the ninth century onward -- for a thousand years -- a state ruled by Zaydi religious leaders and politicians existed in northern Yemen. Then, in 1962, Egyptian-trained Yemeni military officers toppled the Zaydi monarchy and replaced it with a republic. Because of their ties to the ancient regime, Zaydis were perceived as a threat to the new government and were subjected to severe repression.

Nearly three decades later, in 1990, the region known as South Yemen merged with North Yemen to become the Republic of Yemen. Zaydis remained a majority in the north and west of the country, and also in the capital city of Sanaa. However, in terms of the overall population, they became a minority.

According to a 2010 CIA estimate, 65 percent of Yemen's people are Sunnis and 35 percent are Shiites. The majority of those Shiites are Zaydis. Jews, Bahais, Hindus and Christians make up less than 1 percent of inhabitants, many of whom are refugees or temporary foreign residents.

To reduce the dominance of Zaydis in the north, government authorities encouraged Muslims belonging to two Sunni branches with links to Saudi Arabia -- Salafis and Wahhabis -- to settle in the heart of the Zaydis' traditional territories.

Start of Houthi insurgency

Contributing to this trend, in the early 1990s, a Yemeni cleric founded a teaching institute in the Zaydis' heartland. This cleric, educated in Saudi Arabia, developed a version of Salafi Islam.

His institute proselytized with the goal of reforming Muslims through conversion. It educated thousands of Yemeni students and, in less than three decades, the new religious group grew large enough to compete with older groups such as the Zaydis.

According to scholar Charles Schmitz, the Houthi insurgency began in the early 1990s, spurred, in part, by Zaydi resistance to growing Salafi and Wahhabi influence in the north.

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