The 6th (Poona) Division of the British Indian Army, under Major-General Charles Townshend, had fallen back to the strategic town of Kut after the failure to capture Ctesiphon, arriving there around December 3, 1915. The division had lost around a third of its men at Ctesiphon and only around 11,000 soldiers plus cavalry reached Kut. The town was contained within a long loop of the river and when the pursuing Ottoman forces arrived on December 7 the Allied forces were trapped, although much of the cavalry was ordwered to leave and did escape. The Ottoman forces numbered around 11,000 men and were commanded by Nur-Ud-Din. When they failed to take the town in three attacks over December they laid siege.
Townshend was anxious to break-out and then withdraw northwards but the Commander-in-Chief of the MEF John Nixon saw value in tying down the Ottoman forces in a siege. However, when Townshend reported only a few months worth of food a rescuing force was quickly raised.
The British dispatched a relief expedition of 19,000 under Fenton Aylmer from Ali Gharbi in January 1916. It was badly mauled in three clashes in January (Sheikh Sa'ad on the 4th, Wadi on the 13th and Hanna on the 21st), the Ottoman army around Kut had called upon Khalil Pasha, who replaced Nur-Ud-Din as commander on January 10 and brought a further 20-30,000 veteran soldiers into the area. Following the January losses the British dug in but when Nixon was replaced as MEF commander by Percival Lake in February Aylmer's force was reinforced and had one last push, attacking the Dujaila redoubt on March 8 where nothing was achieved for the loss of 4,000 men. Aylmer was dismissed and replaced with George Gorringe on March 12.
The relief attempt by Gorringe is usually termed the First Battle of Kut. The British had gathered around 30,000 soldiers, roughly equal to the Ottoman forces. The battle began on April 5 and the British soon captured Fallahiyeh but with heavy losses, Bait Asia was taken on April 17 but the continuing British losses were making further success unlikely. The final effort was against Sannaiyat on April 22 and cost another 1,200 casualties.
The relief efforts had failed at a cost of around 23,000 Allied killed or wounded to about 10,000 Ottoman casualties. Following a final hopeless effort to send a armoured supply ship the beleaguered Allied forces in Kut were doomed. Townshend arranged an ceasefire on the 26th and after failed negotiations he surrendered on April 29 after 147 days. Around 8,000 Allied soldiers survived to be made prisoners, many were already weakened by illness and almost a third of them died during their captivity.
The British efforts in Mesopotamia were greatly rearranged after the humiliation. General Frederick Stanley Maude was sent to take command in August 1916 and the operations were more closely tied to the British command rather than run from India. Kut was recaptured in February 1917 and the Allied forces went on to take Baghdad in March.