Following the wave of declarations of war between the major European powers at the end of July and early August, Britain declared war on Germany on 5 August. "Prinz Heinrich" was reactivated for wartime service the same day, and she then went into the shipyard in Kiel for preparation work. She was then assigned to the defenses of the port in advance of an expected British attack on Kiel on 25–26 September. When that failed to materialize, "Prinz Heinrich" was assigned to III Scouting Group, part of the High Seas Fleet. From 8 November to 14 April 1915, "Prinz Heinrich" was primarily occupied with guard duty in the Jade Bay and the river Ems. During this period, she participated in the second major German offensive in the North Sea, the operation to bombard Hartlepool on 15–16 December 1914. "Prinz Heinrich", along with and a flotilla of torpedo boats, was assigned to the van of the High Seas Fleet, commanded by Admiral Friedrich von Ingenohl. The main fleet was providing distant cover to Rear Admiral Franz von Hipper's battlecruisers, which were conducting the bombardment. During the night of the 15th, the German battle fleet of some twelve dreadnoughts and eight pre-dreadnoughts came to within of an isolated squadron of six British battleships. However, skirmishes between the rival destroyer screens in the darkness convinced von Ingenohl that he was faced with the entire Grand Fleet. Under orders from Kaiser Wilhelm II to avoid risking the fleet unnecessarily, von Ingenohl broke off the engagement and turned the battle fleet back toward Germany.