The British needed the oil for the Royal Navy so that it could replace its coal-fired vessels. When war broke out in 1914, the Germans, who had previously been partners in the enterprise, turned into the enemy, so control of these oil supplies became one of Britain's primary goals. Under the guise of protecting India's borders with Mesopotamia, an expeditionary force arrived in November 1914 with the aim of taking possession of the towns of Basra, Baghdad and Mosul, the capitals of the provinces bearing the same names. At about the same time, Kuwait became a British protectorate after negotiations instigated by Winston Churchill.
Once British interests in the oilfields had been secured, it was decided to consolidate our position by moving along the river banks to Baghdad which ultimately fell in 1917 but not without bloodshed. The Germans tried to foster a Jihad or holy war against the British forces and Turkey itself offered strong resistance to these incursions into its Empire.
The British Force was composed of the Indian Army (which at that time included a number of British Regular and Territorial units) and was totally unprepared for the kind of guerrilla warfare which it ultimately faced. It was poorly-equipped and under-trained and caught out by a nightmare logistical scenario. As if that wasn't enough, there were flies, mosquitoes, constant high temperatures and humidity. Sickness and disease spread out of control through the camps and the death rate rose to the point where units simply did not have enough officers and men to continue fighting.
The 6th Battalion of the 1st Devonshire Regiment had arrived at Karachi on 11 November 1914 and was attached to the 36th Indian Brigade on 5 January 1916, but by that time, the British had suffered a resounding defeat (November 1915) and had been forced into a frantic retreat to Kut-al-Amara. Over 9000 Indian and British troops were besieged there and were ultimately forced to surrender - probably the British Army's biggest defeat ever..
When Kut fell, the War Office realised (rather late in the day) that the leadership and tactics of the British Army up to this point had been woefully inadequate. A new commander took over - Major General Stanley Maude - and he employed fresh tactics which ultimately led to a decisive defeat of the Turks in February 1917 and the capture of Baghdad the following month - on the very same day that the Berlin-Baghdad railway was captured.
On the night of 13/14 December 1916, the British counter-attack was launched on both banks of the Tigris river. 50,000 men, split into two corps, and including the 6th Devons, slowly made their way through heavy rain and mud along the river banks, taking over 2 months to clear resistance below Kut. On 17 February 1917, Maude launched an attack on both flanks which forced the Turks to retreat from Kut and signalled the end of their resistance. Wasting no time, Maude continued with his advance and went on to recapture Baghdad four weeks later.