On October 3, 1932, Iraq became an independent state and took its seat in the League of Nations, the first of the Class A mandates to get there. Britain had run the country under a mandate since 1920 and had put Faisal I on the throne in 1921. The treaty of 1930 ended the mandate in exchange for a 25-year alliance, air bases at Habbaniya and Shaibah, and British control of military training and supply.
The country had been assembled out of three Ottoman provinces: the Kurdish north around Mosul, the Sunni Arab centre around Baghdad and the Shia south around Basra. The British put them together in 1921 partly because they had all been taken during the Mesopotamia campaign and partly because no alternative arrangement could be agreed. The League awarded Mosul to Iraq in 1925 after Turkey claimed it.
The revolt of 1920 set the terms. A rising that began in the Shia south that summer and spread to the middle Euphrates cost the British several hundred dead and the Iraqi side several thousand, and it convinced London of one thing. Direct rule was too expensive. The mandate that followed gave Britain control through the air instead, which was cheaper: the RAF could bomb a village for a fraction of what garrisoning it cost.
Independence changed who held the state rather than how it was held. The Sunni Arab officers and officials who had come out of the Ottoman army and the Sharifian revolt kept it, and the governments of the 1930s were made and unmade by a small group of them. The Iraqi army massacred the Assyrian minority in the north in 1933. Kurdish revolts ran through the decade.
Britain kept the bases. It used them. In May 1941, after the coup of Rashid Ali al-Gaylani, British forces fought their way back into the country and stayed until 1947. The treaty was renegotiated in 1948 and torn up in 1958, when the monarchy was overthrown. Oil, which began flowing in 1934 at about four million tons a year, is what made the arrangements worth having.