The Division came into existence of Army Order No. 324, issued on 21st August 1914, authorising the formation of the six new Divisions of K1. It was formed of volunteers, under the care of the War Office. The infantry brigades began to assemble on Salisbury Plain. 40th Brigade moved to Chiseldon and Cirencester in September 1914; 39th went to Basingstoke in January 1915. Towards the end of February the entire Division concentrated at Blackdown in Hampshire.

On 7 June 1915, orders were received to prepare to move to the Mediterranean. All mechanical transport was withdrawn, and the first reinforcement drafts were ordered not to sail (other than those for the artillery, and RE Companies).

13 June 1915 : first transports left port, and moved to Alexandria. By 4 July, all units had moved to Mudros, preparatory for landing on Gallipoli.

6-16 July 1915 : Divisional infantry landed on Cape Helles and relieved 29th Division. They left and returned to Mudros at the end of the month, and the entire Division landed at Anzac Cove between 3-5 August 1915.

• The Division took part in the following actions on Gallipoli:

  • The Battle of Sari Bair, 6-10 August 1915

  • The Battle of Russell's Top, 7 August

  • The Battle of Hill 60, ANZAC, 27-28 August

  • Soon afterwards the Division was transferred from ANZAC to Suvla Bay. It was evacuated from Suvla 19-20 December 1915, whereupon the infantry moved after a weeks rest to the Helles bridgehead.

  • The last Turkish attacks at Helles, 7 January 1916

On 8-9 January 1916, the Division was evacuated from Helles, and by 31 January was concentrated at Port Said. The Division held forward posts in the Suez Canal defences.

12 February 1916 : began to move to Mesopotamia, to strengthen the force being assembled for the relief of the besieged garrison at Kut al Amara. By 27 March, the Division had assembled near Sheikh Sa'ad and came under orders of the Tigris Corps. 

• It then took part in the following attempts to relieve Kut:

  • The Attack on the Dujaila Redoubt

  • The Attack at Fallahiyeh

  • The Attack at Sannaiyat

  • The Battle of Bait Aisa

  • The Third Attack at Sannaiyat.

After these efforts failed and Kut fell, the British force in the theatre was built up and reorganised. 

• The Division took part in the following, more successful, operations:
  • The Battle of Kut al Amara, December 1916-February 1917
  • The capture of the Hai Salient, 25 January - 5 February 1917
  • The capture of Dahra Bend, 9-16 February 1917
  • The passage of the Diyala, in the pursuit of the enemy towards Baghdad, 7-10 March 1917

At 10.30am on 11 March 1917, D Squadron, 1/1st Hertfordshire Yeomanry and the 6th (Service) Bn, the King's Own were the first British troops to enter Baghdad, which fell on this day.

During the rest of March and April 1917, operations were undertaken to consolidate the position won at Baghdad, by pushing north across Iraq. As part of "Marshall's Column", the Division fought at Delli 'Abbas (27-28 March), Duqma (29 March), Nahr Kalis (9-15 April), crossed the 'Adhaim (18 April) and at Shatt al 'Adhaim (30 April).

It also fought later in the year, at

The Second and Third Actions of Jabal Hamrin (18-20 October and 3-6 December 1917), and finally at Tuz Khurmatli (29 April 1917).

By 28 May 1918, Divisional HQ had moved to Dawalib, and it remained here until the end of the war. In this inhospitable place, men endured summer temperatures as high as 111 degrees F in the shade. Many working parties were supplied for work on maintaining roads.

On 1 July 1918, Division received orders to detach 39th Brigade for the North Persia Force. It left the Division between 10 July and 19 August 1918. Brigade HQ arrived in Baku at Dunsterforce HQ on 24 August 1918.

In October and early November 1918, parts of 40th Brigade and the Divisional artillery took part in operations as part of "Lewin's Column", pushing north towards Turkey, with advance units reaching as far as Altun Kopri when Turkey signed an Armistice on 31 October 1918.

By 31 December 1918, all areas north of Kirkuk had been evacuated. On 11 January 1919, the Division - by now only some 12,000 strong - began to move south to Amara, and disbandment of the Division proceeded there during February 1919.

6th (Service) Bn, the East Lancashire and 6th (Service) Bn, the Loyal North Lancs were selected for the Army of Occupation in Mesopotamia and were posted to join 34th Indian Infantry Brigade.

13th (Western) Division, the only wholly British Division to have served in Mesopotamia, ceased to exist on 17 March 1919. During the war it had suffered 12,656 killed, wounded and missing, and 57,667 went sick (most of whom returned to duty, and this figure will include men who reported on more than one occasion).

• 38th Brigade

  • 6th (Service) Bn , the King's Own (joined August 1914)

  • 6th (Service) Bn, the East Lancashire (joined August 1914)

  • 6th (Service) Bn, the South Lancashire (joined August 1914)

  • 6th (Service) Bn, the Loyal North Lancs (joined August 1914)

  • 38th Machine Gun Company MGC (joined 24 October 1916)

  • 38th Supply & Transport Column ASC (formed January 1917, left to merge into Divisional Train 1 August 1918)

  • 38th Trench Mortar Battery (G Battery joined from 39th Brigade 7 October 1917, and renamed 38th Battery February 1918)

  • 38th SAA Section (joined March 1918)
    

• 39th Brigade

  • 1 July 1918 : Brigade received orders to be detached from Division and to be attached to the North Persia Force. It left the Division between 10 July and 19 August 1918. Brigade HQ arrived in Baku at Dunsterforce HQ on 24 August 1918.

  • 9th (Service) Bn , the Royal Warwicks (joined August 1914)

  • 7th (Service) Bn, the Gloucesters (joined August 1914)

  • 9th (Service) Bn, the Worcesters (joined August 1914)

  • 7th (Service) Bn, the North Staffords (joined August 1914)

  • 39th Machine Gun Company MGC (joined 26 October 1916)

  • 39th Supply & Transport Column ASC (formed January 1917)

  • 39th Trench Mortar Battery (joined as G Battery 13 January 1917, moved to 38th Brigade 7 October 1917. Replaced by H Battery, which arrived from 14th (Indian) Division on 8 October 1917 and was renamed 39th Battery 18 February 1918)

  • 39th SAA Section (joined March 1918)
    

• 40th Brigade

  • 8th (Service) Bn, the Cheshires (joined August 1914)

  • 8th (Service) Bn, the Royal Welsh Fusiliers (joined August 1914)

  • 4th (Service) Bn, the South Wales Borderers (joined August 1914)

  • 8th (Service) Bn, the Welsh (joined August 1914, left January 1915)

  • 5th (Service) Bn, the Wiltshires (joined December 1915)

  • 2/7th Bn, the Hampshire (joined September 1918)

  • 40th Machine Gun Company MGC (joined 24 October 1916)

  • 40th Supply & Transport Column ASC (formed January 1917, left to merge into Divisional Train 1 August 1918)

  • 40th Trench Mortar Battery (joined as I Battery 23 September 1917, renamed 40th battery 18 February 1918)

  • 40th SAA Section (joined March 1918)

---

• Divisional Troops

  • 8th (Service) Bn (Pioneers), the Welsh (joined January 1915 from 40th Brigade)

  • 5th (Service) Bn, the Wiltshires (joined August 1914, left December 1915)

---

• Divisional Mounted Troops

  • C Squadron, 33rd Cavalry (an Indian unit, attached briefly in March 1916)

  • D Squadron, 1/1st Hertfordshire Yeomanry (joined 8 July 1916, left 20 November 1916, rejoined 3 March 1917, left 3 August 1917)

  • 13th Divisional Cyclist Company

---

• Royal Artillery

  • LXVI Brigade RFA

  • LXVII Brigade RFA (left for 10th Division October 1915)

  • LXVIII Brigade RFA (left for 10th Division October 1915)

  • LXIX (H) Brigade RFA (broken up in May 1916)

  • LV Brigade RFA (arrived from 10th Division January 1916)

  • LVI Brigade RFA (arrived from 10th Division January 1916, left July 1916)

  • 13 Heavy Battery RGA (raised for this Division, the Battery was ordered to France on 30 May 1915 as part of XVII Heavy Brigade. On 23 October 1915, it joined 28th Division)

  • 74 Heavy Battery RGA (joined in Mesopotamia 24 August 1916, left for LXVI Brigade 23 November 1916)

  • 91 Heavy Battery RGA (joined for Gallipoli 7 June 1915, left for XCVI Brigade RGA in 1917)

  • 2/104 Heavy Battery RGA (attached February-March and October-December 1917)

  • 157 Heavy Battery RGA (one section was attached January-February 1917)

  • 157 Siege Battery RGA (attached briefly in February 1917)

  • 177 Heavy Battery RGA (joined 25 October 1917, left 29 May 1918)

  • 384 Siege Battery RGA (joined 25 October 1917, left 1 October 1918)

  • 387 Siege Battery RGA (joined 25 October 1917, left 24 March 1918)

  • 26 (Jacob's) Mountain Battery RGA (joined 23 October 1917, left 10 August 1918)

  • 13th Divisional Ammunition Column RFA (joined August 1914, but did not go overseas with the Division. Unlike most other Divisions, each artillery Brigade retained its own Ammunition Column)

---

• Royal Engineers

  • 71st Field Company (joined early 1915)

  • 72nd Field Company (joined early 1915, left for North Persia Force with 39th Brigade Group)

  • 88th Field Company (joined January 1915)

  • 13th Divisional Signal Company (joined August 1914)

---

• Field Ambulances

  • 39th Field Ambulance (joined Oct 1914)

  • 40th Field Ambulance (joined Oct 1914, left for North Persia Force with 39th Brigade Group)

  • 41st Field Ambulance (joined Oct 1914)

---

• 273 Company MGC (formed October-November 1917)

• 13th Divisional Train (Originally 120, 121, 122, 123 Coys. This train did not sail for Gallipoli. It moved to Egypt in November 1915 and joined 28th Division. A Divisional Transport and Supply Column was formed in Mesopotamia. On 1 August 1918, the Brigade Transport and Supply Columns from 38th and 40th Brigade merged with it, at which point this unit once again became known as 13th Divisional Train)

• 10th Field Bakery (joined as first British mobile field bakery, 23 April 1916)

• 31st Field Butchery (joined 23 April 1916)

• 24th Mobile Veterinary Section, AVC (joined August 1914)

• The Division had no Medium or Heavy Trench Mortar Batteries, but had four "Trench Howitzer Batteries" armed with 2-inch mortars. They were numbered 133, 135, 136 and 137. Joined Division January-February 1917.

• 24th Sanitary Section (joined September 1914 but went to Egypt)

• 28th Sanitary Section (joined in Mesopotamia in March 1916)

• 13th Divisional Motor Ambulance Workshop (joined September 1915 but did not move overseas)