The unification of Saudi Arabia was a military and political campaign, by which the various tribes, sheikhdoms, city-states, emirates, and kingdoms of most of the Arabian Peninsula were conquered by the House of Saud, or Al Saud, between 1902 and 1932, when the modern-day Kingdom of Saudi Arabia was proclaimed under the leadership of Ibn Saud, creating what is sometimes referred to as the Third Saudi State, to differentiate it from the Emirate of Diriyah, the First Saudi State and the Emirate of Nejd, the Second Saudi State, also House of Saud states.
Following the Diriyah agreement between Muhammad ibn Abdul Wahhab and Muhammad ibn Saud, the Al Saud clan founded the First Saudi State, a state based on the strict defense of Islam. The ideology born of this period was later dubbed Wahhabism. Originating in the Nejd region of central Arabia, the First Saudi State conquered most of the Arabian Peninsula, culminating in the capture of the Muslim holy city of Mecca in 1802.
The Al Saud survived in exile and went on to found the Second Saudi State, which is generally considered to have lasted from Turki ibn Abdallah's capture of Riyadh (which he designated as the new capital) in 1824 until the Battle of Mulayda in 1891. The Second Saudi period was marked by instability, which the Al Rashid clan of Jebel Shammar were able to exploit. The Saudi leader, Abdul Rahman ibn Faisal, sought refuge in Ottoman Iraq in 1893.
In January 1902, Ibn Saud and his men reached Riyadh. With only a small force, he felt that the only way to take the city was to capture Masmak fort and kill Ibn Ajlan, Chief of Riyadh, and having achieved these goals they successfully took the city within the night. With the capture of his family's ancestral home, Ibn Saud proved he possessed the qualities necessary to be a sheikh or emir: leadership, courage, and luck. This marked the beginning of the third Saudi state. Ibn Saud's dominions became known as the Emirate of Nejd and Hasa, which lasted until 1921.
In 1913, Ibn Saud, with support from the Ikhwan, conquered al-Hasa from an Ottoman garrison, who had controlled the area from 1871. He then integrated al-Hasa and Qatif into the Emirate. The people in these areas were Shias and the Saudis Wahhabi puritans, resulting in harsh penalties for Shi'a Islam in Saudi Arabia, contrary to what the traditionally tolerant Sunni Ottomans allowed.
Following KuwaitĄNajd War, Ibn Saud imposed a tight trade blockade against Kuwait for 14 years from 1923 until 1937. The goal of the Saudi economic and military attacks on Kuwait was to annex as much of Kuwait's territory as possible. At the Uqair conference in 1922, the boundaries of Kuwait and Najd were set. Kuwait had no representative at the Uqair conference. Ibn Saud persuaded Sir Percy Cox to give him two-thirds of Kuwait's territory. More than half of Kuwait was lost due to Uqair. After the Uqair conference, Kuwait was still subjected to a Saudi economic blockade and intermittent Saudi raiding.
The First Saudi-Hashemite War or the Al-Khurma dispute took place in 1918–1919 between Abdulaziz Ibn Saud of the Emirate of Nejd and the Hashemites of the Kingdom of Hejaz. The war came within the scope of the historic conflict between the Hashemites of Hejaz and the Saudis of Riyadh (Nejd) over supremacy in Arabia. It resulted in the defeat of the Hashemite forces and capture of al-Khurma by the Saudis and his allied Ikhwan, but British intervention prevented the immediate collapse of the Hashemite kingdom, establishing a sensitive cease-fire, which would last until 1924.
Ikhwan raids on Transjordan were a series of plunders by the Ikhwan, irregular Arab tribesmen of Najd, on Transjordan between 1922 and 1924. Though the raids were not orchestrated by Ibn Saud, the ruler of Nejd, nothing was done by him to stop the raiding parties of his ally Ikhwanis. This however changed after the conquest of Hejaz, when the increasingly critical and negative stance of Ibn Saud on Ikhwan raids developed into an open feud and essentially a bloody conflict since 1927.
As Saudi expansion slowed in the 1920s, some among the Ikhwan pushed for continued expansion, particularly to the British-controlled territories such as Transjordan to the north - where the Ikhwan raided in 1922 and 1924. By this time, the few parts of central Arabia that hadn't been overrun by the Saudi-Ikhwan forces had treaties with Britain, and Abdul Aziz was sober enough to realize the folly of a potential conflict with the British. However, the Ikhwan had been taught that all non-Wahhabis were infidels. Faisal al-Dawish of the Mutair tribe and Sultan bin Bajad of the Otaiba tribe, the leaders of the Ikhwan, were among those who accused Abdul Aziz of going "soft", with the former reportedly telling the latter that the Saudis were "as much use as camel bags without handles".