On September 23, 1932, Abdulaziz ibn Abdul Rahman al Saud issued a decree renaming the Kingdom of Hejaz and Nejd as the Kingdom of Saudi Arabia, and the third state the House of Saud had built came into being. He was about 57 and had been at it for thirty years. The decree set out the succession, the flag and the state's adherence to the sharia, and it named his son Saud as crown prince.
The conquest had started with 40 men. In January 1902 Ibn Saud, who had grown up in exile in Kuwait after his family was driven out of Riyadh, took the Masmak fortress in Riyadh with a small party and killed the Rashidi governor. He spent the next decade retaking the Najd, and then in 1913 took al-Hasa from the Ottomans. The First World War complicated things: the British supported Hussein bin Ali, the sharif of Mecca, who revolted against the Ottomans in 1916, and Ibn Saud was paid a subsidy and told not to attack him.
He attacked anyway. The Ikhwan, the tribesmen settled in agricultural colonies and taught a puritan form of Islam, were his fighting force and also his problem: they raided into Iraq, Transjordan and Kuwait, which brought British pressure. Ibn Saud defeated the Rashidis in 1921, took the Hejaz in 1924 and 1925, and had himself proclaimed King of Hejaz and Sultan of Nejd in 1926. The Ikhwan rebelled in 1929 and were destroyed at the Battle of Sabilla in March, by men with machine guns and armoured cars.
What held it together was an alliance made in 1744 between Muhammad ibn Saud and Muhammad ibn Abd al-Wahhab, and renewed in every generation since. The state's religious character is not incidental to it, and the conquest of Mecca and Medina gave the dynasty the custodianship of the two holy places, which is the basis of its claim to leadership in the Islamic world.
The money came later. Saudi Arabia was one of the poorest countries on earth until 1938, and the state's revenue was the pilgrimage and customs. Standard Oil of California found oil in commercial quantities at Dammam No. 7 in March 1938, and the first exports went out in 1939. Production was negligible until the 1950s.
Ibn Saud died in 1953, and the succession has passed among his sons ever since, of whom he had about 45 and perhaps 120 children. The country is now the largest oil exporter in the world, and the state religion and the oil revenue are the two things that have not changed. September 23 is national day.