At the outbreak of war in July 1914, "Schleswig-Holstein" was assigned to guard duty in the mouth of the Elbe River while the rest of the fleet mobilized. In late October, she and her sisters were sent to Kiel to have improvements made to their underwater protection system to make them more resistant to torpedoes and mines, after which II Battle Squadron rejoined the fleet. The squadron covered Rear Admiral Franz von Hipper's battlecruisers of the I Scouting Group while they bombarded Scarborough, Hartlepool, and Whitby on 15–16 December 1914. During the operation, the German battle fleet of some 12 dreadnoughts and 8 pre-dreadnoughts came to within of an isolated squadron of six British battleships. However, skirmishes between the rival destroyer screens convinced the German commander, Admiral Friedrich von Ingenohl, that he was confronted with the entire Grand Fleet, and so he broke off the engagement and turned the fleet for home. In April 1916, the ship had two of her 8.8 cm guns removed and replaced with 8.8 cm Flak guns.