Westfalen was laid down at AG Weser in Bremen on 12 August 1907, launched nearly a year later on 1 July 1908, and commissioned into the High Seas Fleet on 16 November 1909.

The ship was equipped with a main battery of twelve 28 cm (11 in) guns in six twin turrets in an unusual hexagonal arrangement.

In 1919, following the scuttling of the German fleet in Scapa Flow, Westfalen was ceded to the Allies as a replacement for the ships that had been sunk.

Over the next two years, the design was refined into a larger vessel with twelve of the guns, by which time Britain had launched the all-big-gun battleship HMS Dreadnought.

[11] As with her sister Nassau, construction proceeded swiftly and secretly; detachments of soldiers guarded both the shipyard and the major contractors who supplied building materials, such as Krupp.

However, the water level in the Weser River was low at this time of year, so six pontoons had to be attached to the ship to reduce her draft.

[11] On 16 October 1909, before she was commissioned into the fleet, Westfalen along with her sister Nassau participated in a ceremony for the opening of the new third entrance in the Wilhelmshaven Naval Dockyard.

The navy had intended to transfer the ship to II Battle Squadron, but this plan was discarded after the outbreak of World War I in July 1914.

[11] The first operation was conducted primarily by Rear Admiral Franz von Hipper's battlecruisers; the ships bombarded the English coastal towns of Scarborough, Hartlepool, and Whitby on 15–16 December 1914.

The assembled German fleet included Westfalen and her three sister ships, the four Helgoland-class battleships, the battlecruisers Von der Tann, Moltke, and Seydlitz, and several pre-dreadnoughts.

[16] On 16 August 1915, a second attempt was made to enter the Gulf: Nassau and Posen, four light cruisers, and 31 torpedo boats managed to breach the Russian defenses.

[18] Admiral Hipper later remarked that "to keep valuable ships for a considerable time in a limited area in which enemy submarines were increasingly active, with the corresponding risk of damage and loss, was to indulge in a gamble out of all proportion to the advantage to be derived from the occupation of the Gulf before the capture of Riga from the land side."

A bombardment mission followed two days later; Westfalen joined the battleship support for Hipper's battlecruisers while they attacked Yarmouth and Lowestoft on 24–25 April.

[26] Scheer had by this time called for maximum speed in order to pursue the British ships; Westfalen made 20 knots (37 km/h; 23 mph).

[31] Westfalen again assumed a position guiding the fleet, this time because Scheer wanted lead ships with greater protection against torpedoes than the pre-dreadnoughts had.

[37] Broke was engaged by other German warships, including the cruiser Rostock; she was hit at least seven times and suffered 42 dead, six missing, and 34 wounded crew members.

[40] Despite the ferocity of the night fighting, the High Seas Fleet punched through the British destroyer forces and reached Horns Reef by 4:00 on 1 June.

[41] With Westfalen in the lead,[42] the German fleet reached Wilhelmshaven a few hours later, where the battleship and two of her sisters took up defensive positions in the outer roadstead.

As only two of the four German battlecruisers were still in fighting condition, three dreadnoughts were assigned to the Scouting Group for the operation: Markgraf, Grosser Kurfürst, and the newly commissioned Bayern.

[46] However, at 06:00 on 19 August, Westfalen was torpedoed by the British submarine HMS E23, some 55 nautical miles (102 km; 63 mi) north of Terschelling.

Three torpedo-boats were detached from the fleet to escort the damaged ship back to port; Westfalen made 14 kn (26 km/h; 16 mph) on the return trip.

The ship did not actively take part in Operation Albion in the Baltic, though she was stationed off Apenrade to prevent a possible British incursion into the area.

[49] On 22 February 1918, Westfalen and Rheinland were tasked with a mission to Finland to support German army units to be deployed there.

The task force reached the Åland Islands on 5 March, where they encountered the Swedish coastal defense ships Sverige, Thor, and Oscar II.

[49] Westfalen remained in Danzig until 31 March, when she departed for Finland with Posen; the ships arrived at Russarö, which was the outer defense for Hanko, by 3 April.

On 11 August, Westfalen, Posen, Kaiser, and Kaiserin steamed out towards Terschelling to support German patrols in the area.

[49] Following the German collapse in November 1918, a significant portion of the High Seas Fleet was interned in Scapa Flow under the terms of the Armistice.

[50] During the internment, a copy of The Times informed the German commander, Rear Admiral Ludwig von Reuter, that the Armistice was to expire at noon on 21 June 1919, the deadline by which Germany was to have signed the peace treaty.

On the morning of 21 June, the British fleet left Scapa Flow to conduct training maneuvers; at 11:20 Reuter transmitted the order to his ships.