The Hashid tribal federation is the second largest (and the strongest) tribal federation in Yemen.  Member tribes of the Hashid Confederation are found primarily in the mountains in the North and Northwest of the country. It was headed by Sheikh Abdullah ibn Husayn al-Ahmar until his death on December 29, 2007 and is headed by his son Sadiq since. Sadiq al-Ahmar launched the Hashid insurgency as part of the revolution against President Ali Abdullah Saleh in May 2011, culminating in the Battle of Sana'a.
Hashid was already a well known "tribe" (sha`b) since the 1st millennium BCE and it was very frequently mentioned in Sabaic inscriptions. Banu Hamdan was mentioned in Sabaic inscriptions as qayls ("chiefs") of Hashid, later Banu Hamdan acquired control over a part of Bakil and finally gave their clan name to a tribal confederation including Hashid and Bakil. In the late 3rd century Banu Hamdan (and, consequently, Hashid and Bakil) switched their alliance to Himyar. Later some "Hamdani" groups migrated to Syria.
Converting to Islam
In the year 622, Muhammad sent Khalid ibn al-Walid to Yemen to call them to Islam. Khaled managed to convert the Najrani and Tihami Yemenis to Islam but he didn't get a warm response from the Hamdani Yemenis of the highlands. So Mohammed delegated the task to Ali bin Abi Talib, who was much more successful in converting the Hamdani Yemenis.
After the death of Mohammed the Hamdan tribe remained Muslim and didn't join the ridda movement.
After Ali, Power Vacuum in Yemen and the Imam Hadi
The Hamdan tribe remained on the side of Ali, even after the martyrdom of Ali and later his sons. The tribes remained on alliance to Ali but didn't oppose the Ummayas or ally themselves with the other Shias.
At that time Yemen was experiencing a great population movement forming the bulk of the Islamic Expansion mainly settling and Arabizing North Africa/Spain. However, the majority of the Hamdan tribe remained in Yemen which later helped the Hashid/Bakil Hamdani tribes become the biggest local key player, benefiting from the departure of the bulk of the most powerful Nomadic Yemeni tribes of that time into North Africa/Spain in Wetsward movements that continued until the 13th century.
By The 10th century the Imam al-Hadi Yahya bin al-Hussain bin al-Qasim (a scion of Imam al-Hasan, grandson of the Prophet) who, at Sa'da, in 893-7 C.E. arrived to the Northern Highlands on invitation from the Hamdan tribe and from that time till present day the Zaidi moderate Shia teachings became dominant in north Yemen.
Imam Yahya's campaign to subject the country, and more specifically the tribes, to his control, led him to undertake massive campaigns against their influence and power; in fact, his efforts succeeded in permanently eliminating all but two of the ancient confederations (the Bakil is the other one to survive).
Many writers have referred to the Hashid and Bakil confederations as the "two wings" of the Zaidi imamate; in the sense that many of the tribes that belong to these confederations are and were strongly committed to Zaidi Islam, the imams were recognized - to a greater or lesser degree - as the heads of the Zaidi community and could, therefore, count on a measure of support and loyalty. Not all the tribes, however, accepted the temporal and even legal role that the imams arrogated to themselves; consequently, many imams (Imam Yahya and Imam Ahmad in the twentieth century included) complained bitterly about the tribes' inordinate political power.
- Popular Protest in North African and the Middle East(II): Yemen Between Reform and Revolution
- Paul Dresch, A History of Modern Yemen (Cambridge University Press, 2000)
- Andrey Korotayev. Pre-Islamic Yemen. Wiesbaden: Harrassowitz Verlag, 1996.
-  Hamdani tribes that remained in Yemen
- Almsaodi, Abdulaziz. Modern history of Yemen