Because Britain depended on oil from Iraq, and from "Persia" (what Winston Churchill insisted on calling Iran), they could not afford an interruption to supply. Even in 1940 and 1941, anti-British Arabs were sympathetic to Fascists. The Grand Mufti of Jerusalem had a secret plan to cooperate with the Germans and expel the British. Rashid Ali was the leader of the pro-German group in Iraq. The Iraqi army consisted of 50,000 troops commanded by British-trained officers. Rashid Ali became Prime Minister in April 1941. The regent, who was pro-British, Amir Abdul Illah, thought that he would be arrested by Rashid Ali. He fled Iraq by way of Basra, and then ended up in Transjordan.
The British planned to respond by putting a force into Basra, as a starting point. They decided to send a brigade group from the 10th Indian Division. The commander in India was Claude Auchinleck. He would command the Basra operation. This is based on the account in Vol.II of the Australian Official History.