The imams based their legitimacy on descent from the Islamic prophet Muhammad, mostly via the prominent Zaydiyya theologian al-Qasim al-Rassi (d. 860) - his cognomen refers to ar-Rass, a property in the vicinity of Mecca that he owned.

[4] Although he did not succeed in establishing any permanent administrative infrastructure, al-Hadi's descendants became the local aristocracy of the northern highlands, and it is from among them that most of the imams of Yemen were selected for the next one thousand years.

Meanwhile, a multitude of smaller dynasties and families established themselves in the highlands, as well as in Tihama (the low coastal plain) where the imams rarely ruled.

Ironically, the Sunni Rasulids, who eventually concentrated their rule in southern Yemen for precisely that reason, were the dynasty under which the region experienced the greatest economic growth and political stability.

Eventually the Europeans entered the Middle East, specifically the Portuguese and then others, in the effort to control the Red Sea trade.

His son al-Mu'ayyad Muhammad managed to gather the entire Yemen under his authority, expel the Turks, and establish an independent political entity.

The territory controlled by the imams shrank after the late 17th century, and the lucrative coffee trade declined with new producers in other parts of the world.

The Qasimid state has been characterized as a "quasi-state" with an inherent tension between tribes and government, and between tribal culture and learned Islamic morality.

While Yemen under the two imams seemed almost frozen in time, a small but increasing number of Yemenis became aware of the contrast between an autocratic society they saw as stagnant and the political and economic modernization occurring in other parts of the world.

In 1965 Egyptian president Gamal Abdel Nasser met with King Faisal of Saudi Arabia to consider a possible settlement to the civil war.

The meeting resulted in an agreement whereby both countries pledged to end their involvement and allow the people of North Yemen to choose their own government.

Fighting continued until 1970, when Saudi Arabia halted its aid to royalists and established diplomatic ties with North Yemen.

The last ruling Rassid descendant Muhammad al-Badr, greatly disappointed by the Saudi recognition of the republic, emigrated to London where he died in 1996.

[12] In June 1974 military officers led by Colonel Ibrahim al-Hamdi staged a bloodless coup, claiming that the government of Al-Iryani had become ineffective.

Saleh strengthened the political system, while an influx of foreign aid and the discovery of oil in North Yemen held out the prospect of economic expansion and development.