The 21st Division was formed in September 1914 as part of Kitchener's Third Army (K3), comprising units mostly from Yorkshire, Northumberland, Durham and Lincolnshire. It was destined to spend its entire period of active service on the Western Front, taking part in almost all the major engagements. Only two weeks after having arrived in France, and with no battlefield experience, they were thrown into action on the second day of the Battle of Loos. Badly misused by high command, it was not surprising that they underperformed.

The division, from May 1916 under the command of Major-General David "Soarer" Campbell, managed to recover from this disastrous baptism of fire to achieve creditable success on three occasions during the Battle of the Somme, including the attack north of Fricourt on the first day. It was during this campaign that the original 63 Bde was exchanged for 110 Bde, the latter's four battalions of the Leicestershire Regiment performing admirably at the Battle of Bazentin on 14 July. The division then re-entered the fray with the newly-introduced tanks in September as the BEF captured the villages of Flers and Gueudecourt. In 1917, they experienced mixed fortunes both at Arras, coming up against the formidable Hindenburg Line defenses, and during the latter stages of Third Ypres as they defended Polygon Wood against German counter attacks before struggling forward through the October mud to assault the village of Reutel.

Between March and June of 1918, the division faced all three major German Spring Offensives: they put up a stout defense of the village of Epéhy on 21 March before conducting a lengthy fighting retreat that reduced its battalions to barely 200 men each. In April, they halted the German advance near Ypres during the Battle of the Lys and then, having been sent to a quiet French sector to rest and reorganize, on 28 May they found themselves in the path of the Blücher offensive and were sent reeling as the Germans stormed across the Chemin des Dames Ridge.

The division survived - barely - and recovered to play its part in the Hundred Days victories. It was involved in a dozen or so attacks through the summer and autumn, recapturing much of the ground ceded during the March retreat before extending their advance across the Selle and Sambre rivers. They fought their last engagement four days before the armistice when they captured the village of Limont-Fontaine.

The 21st had a busy and costly war, losin