The ships of her class were already outdated by the time they entered service, as they were inferior in size, armor, firepower, and speed to the revolutionary new British battleship HMS Dreadnought.

She served with the fleet throughout the first two years of World War I, seeing action at the Battle of Jutland on 31 May – 1 June 1916, where she was briefly actively engaged in combat.

Schlesien saw limited combat during World War II, briefly bombarding Polish forces during the invasion of Poland in September 1939.

The passage of the Second Naval Law in 1900 under the direction of Vizeadmiral (VAdm—Vice Admiral) Alfred von Tirpitz secured funding for the construction of twenty new battleships over the next seventeen years.

[3] Dreadnought's revolutionary design rendered every capital ship of the German navy obsolete, including the Deutschland class.

She was equipped with three triple expansion engines and twelve coal-fired water-tube boilers that produced a rated 18,664 indicated horsepower (13,918 kW) and a top speed of 18.5 knots (34.3 km/h; 21.3 mph).

Her offensive armament was rounded out with a secondary battery of fourteen 17 cm (6.7 in) SK L/40 guns mounted individually in casemates.

His tenure as fleet commander was marked by strategic experimentation, due to the threat the latest underwater weapons posed, and the fact that the new Nassau-class battleships were too wide to pass through the Kaiser Wilhelm Canal.

The year's autumn maneuvers were confined to the Baltic and the Kattegat, during which another fleet review was held in order to prepare for an Austro-Hungarian delegation that included Archduke Franz Ferdinand and Admiral Rudolf Montecuccoli.

In mid-1912, due to the Agadir Crisis, the summer cruise was confined to the Baltic, to avoid exposing the fleet during the period of heightened tension with Britain and France.

[14] During the operation, the German battle fleet of 12 dreadnoughts and 8 pre-dreadnoughts came to within 10 nmi (19 km; 12 mi) of an isolated squadron of six British battleships.

Skirmishes between the rival destroyer screens convinced Admiral Friedrich von Ingenohl that he was confronted with the entire Grand Fleet, and so he broke off the engagement and went back home.

[8] On 24–25 April, Schlesien and her sisters joined the dreadnoughts of the High Seas Fleet to support the battlecruisers of I Scouting Group on a raid of the English coast.

[17] Admiral Scheer immediately planned another foray into the North Sea, but the damage to Seydlitz delayed the operation until the end of May.

[22] Scheer decided to reverse the course of the fleet with the Gefechtskehrtwendung, a maneuver that required every unit in the German line to turn 180° simultaneously.

Mauve considered moving his ships to the rear of the line, astern of III Battle Squadron dreadnoughts, but decided against it when he realized the movement would interfere with the maneuvering of Admiral Franz von Hipper's battlecruisers.

[28] The British battlecruisers scored several hits on the German ships; in the brief melee a near miss from a large-caliber gun sprayed shell splinters onto Schlesien's decks, killing one man and wounding another.

[29] Late on the 31st, the fleet organized for the night return to Germany; Schlesien and Schleswig-Holstein fell in behind the mauled battlecruisers Von der Tann and Derfflinger at the rear of the line.

[31] The German fleet reached Wilhelmshaven a few hours later, where the undamaged dreadnoughts of the Nassau and Helgoland classes took up defensive positions.

[33] The experience at Jutland demonstrated that pre-dreadnoughts had no place in a naval battle with dreadnoughts, and they were thus left behind when the High Seas Fleet sortied again on 18 August.

[35] In late April, Schlesien was taken into the drydock at the Kaiserliche Werft (Imperial Shipyard) in Wilhelmshaven to be converted into a training ship for naval cadets.

Schlesien initially went to Flensburg, but mutiny had already spread to the port, so on orders from the Reichsmarineamt (Imperial Naval Office) she sailed instead to Swinemünde, where on 10 November her commander, Fregattenkapitän (Frigate Captain) von Waldeye-Hartz hauled down his flag and placed his ship out of service.

In the course of 1928, Schlesien went on a fleet cruise in the Atlantic in July and conducted shooting training in August with the old ironclad, now target ship, Baden.

From 3 April to 6 June, Schlesien embarked on a major training cruise by herself to the Mediterranean Sea, during which she visited Messina, Sicily, where her senior officers met King Victor Emmanuel III of Italy.

Kommodore (Commodore) Max Bastian replaced Foerster in September 1932, and at the same time, Kapitän zur See (Captain at Sea) Wilhelm Canaris, who later served as chief of the Abwehr, took command of the ship, a position he would hold for the next two years.

Schlesien was converted back into a training ship for cadets between 18 February and 8 April 1935, and she was formally removed from the fleet organization on 30 September.

The newer boilers were more efficient, which allowed fewer of them to be used; the additional space this created was used as crew compartments for the cadets and an instruction room.

In January 1941, she was reactivated to again serve as an icebreaker in the Baltic until 31 March, when she was again decommissioned in Gotenhafen, thereafter being used as a stationary training hulk with a skeleton crew.

The Soviet fleet did not attempt to leave port after Germany invaded in June, and so Schlesien was detached from the force guarding the minefields in October.

Early on the morning of 3 May, while steaming south of Greifswalder Oie, she struck a British air-dropped naval mine; the explosion killed two men and caused significant flooding.