NATURE GAVE THE Gulf two bounties. Above, in the shallow waters of the sea, oyster beds have yielded pearls since antiquity. In the 19th century they were sold across the world through merchants in Mumbai. Below ground, in ripples of rock where the Arabian plate pushes under the Eurasian one, some of the world’s richest oil deposits were discovered in the early 20th century—in Iran in 1908, Iraq in 1927, Bahrain in 1932 and, above all, in Saudi Arabia in 1938.

Until then the Gulf had mostly been of marginal importance to surrounding empires. Notions of sovereignty were vague and applied to people more than land. Nomadic, tribal lifestyles meant that rulers had limited means to enforce their will, since dissenters could move elsewhere.

The Al Khalifas who settled in Bahrain split from the Al Sabahs of Kuwait; both trace their origins to a migration from central Arabia in the 17th century. The Al Thanis of Qatar, in turn, rebelled against the Al Khalifas. The Al Maktoums of Dubai split away from the Al Nahyans of Abu Dhabi. Tribes straddle modern borders, and many of the Gulf’s ruling clans intermarried.

Starting in the 18th century, the Gulf emirs came under the protection of the British empire, which sought to keep rivals away from the approaches to India. The British imposed truces on the internecine fights of the emirs (hence the region’s name, “the trucial coast”).

In the Arabian interior, meanwhile, the Al Sauds in the Nejd struck a pact in the 18th century with a puritanical cleric, Muhammad ibn Abdel-Wahhab, and his followers. Together they conquered much of the peninsula and the alliance persisted through the ebb and flow of the Al Sauds’ rule.

The first Saudi state of 1744-1818 was crushed by the Ottomans. The weak second Saudi state of 1824-91 collapsed from internal turmoil. The third and current one was reconstituted by Abdel Aziz Al Saud and named Saudi Arabia in 1932. He received some help from Britain. It acquiesced when he defeated the Hashemites, Britain’s ally in the first world war, and took the holy cities of Mecca and Medina. But Britain prevented him from overwhelming, among other places, the trucial coast and Oman.

President Franklin Roosevelt met King Abdel Aziz in Egypt in 1945 after the Yalta summit, laying the foundation for an enduring, if troubled, alliance. The British withdrew from east of Suez in 1971 and were replaced by America as the region’s protector from 1979, after the Iranian revolution and the Soviet intervention in Afghanistan.

Oil turned the poor and loosely organised tribes of the Arabian peninsula into some of the world’s wealthiest states. In contrast with most countries, which must tax citizens to raise the money needed to provide services and security, Gulf rulers collected rents from oil and distributed some of the bounty to citizens. These “rentier states” provided cradle-to-grave benefits in return for obedience. In Saudi Arabia, the rulers gave Wahhabi clerics leeway to impose social norms and in return received religious legitimacy for their rule.

Gulf citizens were propelled from poverty to a life of comfort. Standards of health and education improved quickly. The thinly populated Gulf states hired Western experts to help them build their countries and an army of Asians to do the menial labour. Foreigners make up about half the population of Gulf states, ranging from 90% in the UAE and Qatar to 30% in Saudi Arabia.

For all the benefits of oil, however, some lamented the passing of the hardy desert life. Bandar bin Surur, a 20th-century Saudi poet, wrote: “The chiefs of Nejd’s tribes who uprooted mighty armies/have become docile and content to feed, like chickens, on grain thrown to them on dirt.”

Because benefits for citizens are so generous, rentier states define citizenship narrowly, even cruelly, excluding hundreds of thousands of Arabs known as bidoon, who are not deemed to qualify. Foreign workers, too, are largely kept in a state of bondage through the kefala system, whereby local sponsors of expatriates must give consent for them to change jobs or leave the country. As with much else, citizenship in the Gulf is a gift from the ruler, not a right.