The Iraq coup of 1941, also known as the Rashid Ali Al-Gaylani coup.

In May 1941, four Iraqi nationalist army generals ("the Golden Square") overthrew the regime of the Regent and installed Raschid Ali as Prime Minister. The Golden Square had acted in cahoots with Nazi German intelligence and propaganda activities in the area with the objective of securing Iraqi oil rescources for the Axis and removing the British bases at Habbaniya and Shaibah.

Britain reacted by landing troops at Basra which it claimed it was entitled to do under the mutual defence treaty. The new Iraqi government then moved substantial ground forces to besiege the large RAF training base at Habbaniya. This was taken by the base commander as an act of aggression, and using the obsolete training aircraft jury rigged for the stacks of ancient ordnance on the base, and flying instructors and students as aircrew, the scratch force took the war to the encamped Iraqi forces, and when with assistance from the small ground forces at the base, and the loyal Iraqi levies (Iraqi troops raised by the British for defence of the bases), the Iraqi nationalists retreated to Falluja, the battle was taken to the remaining Iraqi airforces bases.

Habbaniya had lifted the siege with its own resources. Once the reinforcements (British, Palestine, and Arab Legion) arrived in two columns (Habforce and KingCol) across the desert from Palestine and TransJordan, the Iraqi nationalists were cleared from Falluja and pursued along the river valley to Baghdad, which fell within a week with a restoration of the Regent and the pro-British government.

During the course of the Iraq "rebellion", very late reinforcements for the nationalists were received from both Germany and Italy, whose aircraft were crudely overpainted with Iraq colours. The Luftwaffe flew sorties from Mosul against both the base at Habbaniya and the relieving Commonwealth forces moving across from Jordan to little effect.

However, the Vichy French authorities in the Syrian Mandate had given full assistance to both the pro-Axis Iraqi nationalists (handing over stacks of war material) and to the Germans (providing staging bases for the Lufwaffe). Even before the end of the Iraq campaign this had led to RAF attacks on airbases in Syria, and would lead to the full scale invasion of Vichy Syria within weeks.

In view of the potential threat to oil supplies for the allies revealed by the Iraq coup, and the level of pro-Axis activity in Iran, Britain and the USSR invaded that country within months and occupied it until the end of the war.

Sources:

Hidden Victory: The Battle of Habbaniya, May 1941 by Air Vice-Marshal A.G. Dudgeon

The Golden Carpet by Somerset de Chair

The Battle for Habbaniya - The forgotten war RAF Official website http://www.raf.mod.uk/history/opsrep.html