On 16 October 1909, "Nassau" and her sister participated in a ceremony for the opening of the new third entrance in the Wilhelmshaven Naval Dockyard. They took part in the annual maneuvers of the High Seas Fleet in February 1910 while still on trials. "Nassau" finished her trials on 3 May and joined the newly created I Battle Squadron of the High Seas Fleet. Over the next four years, the ship participated in the regular series of squadron and fleet maneuvers and training cruises. The one exception was the summer training cruise for 1912 when, due to the Agadir Crisis, the cruise only went into the Baltic. On 14 July 1914, the annual summer cruise to Norway began. The threat of war caused Kaiser Wilhelm II to cancel the cruise after two weeks, and by the end of July the fleet was back in port. War between Austria-Hungary and Serbia broke out on the 28th, and in the span of a week all of the major European powers had joined the conflict.