The Mesopotamian Campaign was a campaign in the Middle East fought between the Entente, troops from the British Raj (mostly Indian troops), and the Ottoman Empire (Turkey).

British and Turkish interests in the area; The British interests were to protect their oil refinery at Abadan and to defend their allies in the area (Persia and Kuwait). Ottoman interests were to maintain the status quo. Shortly after the war started in Europe, the British sent a military force to protect Abadan, one of the world's earliest oil refineries. The British didn't use much oil at the start of the war but they had already started building warships which would be fueled by oil instead of coal by 1912 so the oil refineries were not unimportant.

British occupation and advance 1914-1915; In 1914 a British force attacked and took the Turkish fort at Fao Landing. Two weeks later, the British occupied the city of Basra. In 1915 the British General Townshend was ordered to advance to Kut or Baghdad if possible. The Ottoman Empire sent an old German general, Baron von der Goltz, to take command of the Ottoman army in the field.

Battle at Ctesiphon 1915; General Townshend and Goltz fought a battle at Ctesiphon, 25 miles south of Baghdad. The battle was inconclusive as both the Ottomans and the British ended up retreating from the battlefield. However, Townshend concluded a full scale retreat was necessary so he withdrew in good order back to Kut, then halted and fortified the position.

Siege of Kut 1915-1916; Defending Kut as opposed to retreating back to Basra was a mistake. Kut was isolated, and while it could be defended, it could not be resupplied. The Turkish forces built defensive positions around the land side of Kut, laid siege to the British, and built fortified positions down river designed to fend off any attempt to rescue Townshend. The British made three major attempts to break the siege, but each effort was unsuccessful. Townshend surrendered April 29, 1916 and his 8,000 soldiers became captives of the Ottomans.

The British viewed the loss of Kut as a humiliating defeat. It had been many years since such a large body of British Army soldiers had surrendered to an enemy. This loss followed only four months after the British defeat at the Battle of Gallipoli. Nearly all the British commanders involved in the failure to rescue Townshend were removed from command. The Turks proved they were good at holding defensive positions against superior forces.

Battle of Baghdad 1916-1917; For the next six months the British trained and organized a new army under the command of General Maude. An offensive was launched on December 13 1916. The British advanced up both sides of the Tigris river, forcing the Ottoman army out of a number of fortified positions along the way. General Maude's offensive was methodical, organized, and successful. The British recaptured Kut in February of 1917, destroying most of the Mesopotamian-based Ottoman army in the process.

On March 11, 1917 the British entered Baghdad. Amidst the confusion of the retreat a majority of the Ottoman army (some 9,000 soldiers) were captured.

End of the War; The British resumed their offensive in late February 1918 capturing Kifri and Hit (previously called Khanaqin). In October the British went on the offensive for the last time and fought a battle at the Battle of Sharqat, routing the Turkish army. General Marshall accepted the surrender of Khalil Pasha and the Turkish 6th Army on October 30 1918. British troops marched unopposed into Mosul on the 14 November 1918. The British lost 92,000 soldiers in the Mesopotamian campaign. Turkish losses are unknown but the British captured a total of 45,000 prisoners of war. The vast majority of the British empire forces in this campaign were recruited from India.