SMS Roon[a] was the lead ship of her class of armored cruisers built for the German Kaiserliche Marine (Imperial Navy) in the early 1900s as part of a major naval expansion program aimed at strengthening the fleet.

The threat of British submarines convinced the German command to withdraw old vessels like Roon by early 1916, and she was again decommissioned and eventually used as a training ship.

The two ships were incremental developments of the preceding Prinz Adalbert-class cruisers, the most significant difference being a longer hull; the extra space was used to add a pair of boilers, which increased power by 2,028 metric horsepower (2,000 ihp) and speed by 0.5 knots (0.93 km/h; 0.58 mph).

[1] The launch of the British battlecruiser HMS Invincible in 1907 quickly rendered all of the armored cruisers that had been built by the world's navies obsolescent.

She carried up to 1,570 t (1,550 long tons) of coal, which enabled a maximum range of up to 4,200 nautical miles (7,800 km; 4,800 mi) at a cruising speed of 12 knots (22 km/h; 14 mph).

Her secondary armament consisted of ten 15 cm (5.9 in) SK L/40 guns; four were in single-gun turrets on the upper deck and the remaining six were in casemates in a main-deck battery.

The ship then began sea trials that lasted until 9 July; she joined I Scouting Group on 15 August, where she replaced the armored cruiser Friedrich Carl as the flagship of the deputy commander, KzS and Kommodore (Commodore) Raimund Winkler.

At the same time as Hoffmann's departure, Winkler also left his post, being replaced by KzS and Kommodore Eugen Kalau vom Hofe, who transferred his flag to Friedrich Carl in October.

[6] She spent the following years participating in training exercises and cruises with the ships of I Scouting Group as well as the entire High Seas Fleet.

This routine was interrupted in early 1907 when the ship was sent to the United States to participate in the Jamestown Exposition, which commemorated the 300th anniversary of the arrival of colonists in Chesapeake Bay.

Two days later, the international fleet, which also included contingents from Great Britain, Japan, Austria-Hungary, France, Italy, and several other nations, held a naval review as part of the exposition.

[9] On 5 March, Roon returned to flagship duty, with now Konteradmiral (KAdm—Rear Admiral) Kalau von Hofe back aboard the ship.

[11][12] KzS Reinhard Koch relieved Jacobsen as the group deputy commander after the annual fleet maneuvers in September 1909 but on 1 October he transferred his flag to Yorck.

[7] Following the outbreak of World War I in July 1914, Roon was mobilized for wartime service on 2 August and was initially assigned to II Scouting Group as the flagship of KAdm Gisberth Jasper.

The following day, Roon and the rest of the group took part in a sortie into the eastern Baltic Sea in a failed attempt to rescue the light cruiser Magdeburg that had run aground in Russian territory.

Along with the armored cruiser Prinz Heinrich, Roon was assigned to the van of the High Seas Fleet, which was providing distant cover to Rear Admiral Franz von Hipper's battlecruisers while they were conducting the bombardment.

Following reports of British destroyers from Roon as well as from Hamburg, Admiral Friedrich von Ingenohl ordered the High Seas Fleet to disengage and head for Germany.

Twenty minutes later, Roon signaled the two light cruisers and ordered them to abandon the pursuit and retreat along with the rest of the High Seas Fleet.

On 11 May, the British submarine E9 spotted Roon and several other ships en route to Libau, which had been recently captured by the German army.

Roon and Lübeck covered a minelaying operation with Albatross on 30 June that lasted through 2 July and resulted in the Battle of Åland Islands.

Augsburg escaped while the destroyers covered the retreat of Albatross, which was severely damaged and forced to seek refuge in neutral Swedish waters.

The loss of Prinz Adalbert three days later to a British submarine convinced the German command that the threat of underwater weapons was too serious to continue to operate older vessels with insufficient protection, including Roon.

[24] The German Navy had previously experimented with seaplane carriers, including the conversion of the old light cruiser Stuttgart early in 1918 for service with the fleet.