British interests in the Persian Gulf were at danger when the Ottoman Empire joined the Central Powers in November 1914. Great Britain had long-standing relationships with the local sheikhs, considerable economic interests, and authority over piracy in the Persian Gulf. The sheikhs of the Gulf welcomed British forces when they arrived in Bahrain late in October despite their lack of affection for the Ottoman government. Tradition has it that British oil interests in Persian Arabistan were the main impetus for the British to intervene in Mesopotamia, but economic investments and the worry that the Turks would start a holy war that would spread to India were just as important—if not more—reasons for going to war with the Turks.
Viceroy Hardinge and General Sir Beauchamp Duff, the commanding officer in India, pushed for an expansion of the campaign to prevent the local Arabs from rising up and believing the British were hesitant to advance because of fear of a holy war. The secretary of state for India in London was persuaded by this argument to approve advances up both the Tigris and Euphrates. The British and Indian forces suffered greatly during these summertime battles, but Nixon, the commander in Mesopotamia, didn't seem to care much. The attraction of Baghdad, another hundred miles up the Tigris, was too great for India and London to refuse after the Sixth Division conquered Amara, 450 miles up the Tigris from Basra.
But there was an issue. Force D forces marched up the rivers farther and farther, but they received supplies less frequently since their transport boats had to travel farther and farther and they had no extra river boats. Their capacity to defend the region they had captured became more and more precarious as they had fewer military and medical equipment available to support themselves. Later, General Nixon in Mesopotamia and the Indian government both asserted that they were aware of the need for extra transportation, but nobody told London. The British Cabinet concluded in October 1915 that the conquest of Baghdad would be extremely important and would assist to allay the disappointment caused by the previous Gallipoli disaster.
The British government took command of the campaign from India and started supplying the required equipment to keep a viable posture for the duration of the conflict. After hearing of the suffering of the troops, a parliamentary commission was formed, which found fault with the Indian leaders but spared no one any punishment. Mesopotamia's conditions got better, but defense was the rule of the day until Sir Stanley Maude, the new commanding general, persuaded the General Staff to allow him to advance. Confident in his supplies and transportation, Maude retook Kut in February 1917 before taking Baghdad in the first few days of March.
Throughout the entire war, the British had sought to either directly or indirectly annex Mesopotamia to their empire. The Ottoman Empire had been partitioned between the French, Russians, and these three nations in secret agreements in 1915. British and French claims in the covert negotiations did not match promises made to the Arabs to win their support against the Turks, as evidenced by the early publication of that agreement by the Soviets when they ended the war in November 1917. As a result, Britain was forced to deny any claims to the region.
References:
Barker, A. J., The Neglected War (London:
Cassel 6k Co., 1967); Davis, Paul, Ends and Means:
The British Mesopotamia Campaign and Commission
(Rutherford, NJ: Fairleigh Dickinson University
Press, 1994).