The Kingdom of Hejaz and Nejd (Arabic: مملكة الحجاز ونجد, Mamlakat al-Ḥijāz wa-Najd), initially the Kingdom of Hejaz and Sultanate of Nejd (Arabic: مملكة الحجاز وسلطنة نجد, Mamlakat al-Ḥijāz wa-Salṭanat Najd), was a dual monarchy ruled by Abdulaziz (Ibn Saud) following the Saudi conquest of Hejaz by the Sultanate of Nejd in 1925. It was the fourth iteration of the Third Saudi State.
In 1932, the two kingdoms were unified as the Kingdom of Saudi Arabia.
The Sultanate of Nejd (Arabic: سلطنة نجد, Salṭanat Najd) was the third iteration of the Third Saudi State, from 1921 to 1926. It was a monarchy led by the House of Saud, and a legal predecessor of modern-day Saudi Arabia. This version of the Third Saudi State was created when Abdulaziz ibn Saud, Emir of Riyadh, declared himself sultan over Nejd and its dependencies. On the 2nd of December 1922, the Nejd signed an agreement with Kuwait defining their border with each other. The border would be a straight line along the 29th parallel. In December 1925, the Kingdom of Hejaz surrendered to the forces of ibn Saud, who was thereafter proclaimed king of the Hejaz in January 1926 and merged his dominions into the Kingdom of Hejaz and Nejd.
Saudis (Arabic: سعوديُّون, romanized: suʿūdiyyūn; local dialects: سعوديين, suʿūdiyyīn) or Saudi Arabians are the citizen population of the Kingdom of Saudi Arabia, who speak the Arabic language, a Central Semitic language, and share a common ancestry, history, and culture. They are mainly composed of Arabs and live in the five historical Regions: Najd, Hejaz, Asir, Tihamah and Al-Ahsa; the regions which the Kingdom of Saudi Arabia was founded on or what was formerly known as the Kingdom of Hejaz and Nejd in the Arabian Peninsula. Saudis speak one of the dialects of Peninsular Arabic, including the Hejazi, Najdi, Gulf and Southern dialects (e.g. Bareqi), or South Arabian languages (e.g. Faifi) as a mother tongue.
The Emirate of Asir, also known as the Idrisid Emirate, was a short-lived state that existed from 1907 until its annexation by Saudi Arabia in 1934. Located in the Yemeni Red Sea coast of South Arabia, it was founded by Muhammad ibn Ali al-Idrisi, the great-grandson of ibn Idris, the founder of the Idrisiyya, a Sufi tariqa of Sunni Islam, in rebellion against the Ottoman Empire. The authority of the Emirate was restricted to an 80 mi (129 km) long strip of the Tihamah region and extending about 40 mi (64 km) inland to the scarp of highland Asir al-Sarah. Its capital was Sabya.
It gained the support of Great Britain during the First World War, and flourished until the death of Muhammad al-Idrisi in 1920. Following the collapse of the Ottoman Empire in 1918, the emirate expanded its domains, reaching as far as Hodeidah. The Emirate was gradually absorbed into the Kingdom of Hejaz and Nejd as a protectorate, and was formally annexed by its successor, Saudi Arabia, under the Treaty of Taif in 1934.