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The tribe is an offshoot of the tribe of Banu Yam, the principal tribe of Najran near Yemen. The 'Ajman separated from Yam sometime in the 18th century, along with another sub-tribe known as Al Murrah. Both tribes migrated towards the east, with the Ajman settling the region between al-Ahsa and Kuwait, and Al Murrah ending up roaming regions further south on the border of the Empty Quarter desert. Unlike their parent tribe of Yam, who are predominantly Ismaili Shi'as, the Ajman and Al Murrah are entirely Maliki Sunni.
The 'Ajman were noted for their strength in battle and were important players in the wars and politics of eastern and central Arabia in the 19th and early 20th centuries. Their most famous leader (or shaikh) during the 19th century was Rakan ibn Hithlayn, who is still well known in Arabian tribal lore, and who was noted for his poetry as well as aptitude in battle against the Ottoman Turks. The 'Ajman, however, were defeated by Faisal ibn Turki, the second Imam of the Second Saudi State, who later married into the tribe. Later on, they supported the cause of the Saud al-Kabir branch of the Al Saud against their cousin Abdul Aziz ibn Saud, the founder of Saudi Arabia.
A section of the Ajman led by Dhaydan ibn Hithlayn joined the Ikhwan movement in 1912, providing military support for Ibn Saud, but later rebelled against him. The Ajman and their allies from the tribes of Utaybah and Mutayr were defeated by Ibn Saud in 1929 in the Battle of Sabilla, which put an end to the Ikhwan rebellion.
Nearly all the Ajman have abandoned nomadic life and have settled in the Persian Gulf states, particularly the eponymous Emirate of Ajman—a member emirate in the United Arab Emirates. There are also many in Saudi Arabia and Kuwait. Their main tribal territory is Joudah, also known as Wadi el-Ajman ("the valley of the Ajman"), located on the road between Riyadh and Dammam.
Ajman attacked Sobyie tribe in 1764 who had to call Ibn Saud to protect them from Ajman tribe. Ibn Saud responded immediately and killed 50 and captured 240 persons of Ajman. Shikh Rakan Bin Hathleen sent two of his sons to Banu Yam in Najran asking them for help. Najran was such nightmare for Ibn Saud that time. Shikh Hassan Bin Hebat Allah was the religious leader of Yam and he responded to Ajman and called for the general mobilization to AD'deryah in Ryadh with 500 men on 500 black horses (one of their techniques in war). Yam reputation was terrifying every single tribe at that time which pushed Qahtan tribe to build an alliance with them. Yam arrived in Ryadh and joined Ajman and moved to Adderyah. Ibn Saud had 3700 men but Shikh Muhammed bin Abdulwahab warned Mohammed bin Saud and ask him to make peace with Yam but he fought them and was defeated. Yam killed about 390 men and captured 220 men and got the Ajman prisoners back from Ibn Saud. Ibn Saud had to make peace with Yam so that Yam will go back to Najran and Ajman will stay in Najd under Yam's full protection. Ibn Saud knows that Yam keep their word and fight for it so that he was not worry about them when they made peace. This battle named Al-Ha'ir (الحائر).