Mandatory Iraq, established in 1921 under British control, represented a pivotal phase in Iraq's modern history. The mandate was a consequence of the Ottoman Empire's dissolution post-World War I and the subsequent division of its territories as per the Treaty of Sèvres in 1920 and the Treaty of Lausanne in 1923.

In 1921, the British installed Faisal I as the King of Iraq, following his involvement in the Arab Revolt against the Ottomans and the Cairo Conference. Faisal I's reign marked the beginning of the Hashemite monarchy in Iraq, which lasted until 1958. The British mandate, while establishing a constitutional monarchy and a parliamentary system, maintained significant control over Iraq's administration, military, and foreign affairs.

The period saw significant developments in Iraq's infrastructure, including the establishment of modern educational institutions, the building of railways, and the development of the oil industry. The discovery of oil in Mosul in 1927 by the British-owned Iraq Petroleum Company significantly impacted the region's economic and political landscape.

However, the mandate period was also marked by widespread discontent and rebellion against British rule. Notable was the Great Iraqi Revolution of 1920, a large-scale uprising that significantly influenced the formation of the Iraqi state. This revolt prompted the British to install a more compliant monarch and ultimately led to Iraq's independence.

In 1932, Iraq gained formal independence from Britain, though British influence remained significant. This transition was marked by the Anglo-Iraqi Treaty of 1930, which allowed for a degree of Iraqi self-governance while ensuring British interests, particularly in military and foreign affairs.

Mandatory Iraq laid the foundation for the modern Iraqi state, but it also sowed seeds of future conflicts, particularly concerning ethnic and religious divisions. The British mandate's policies often exacerbated sectarian tensions, laying the groundwork for later political and social strife in the region.