Restricted by treaty to 10,000 tons and 11" guns for new naval builds, the Weimar Navy sought to modernize its old fleet. A new, long-range 11" rifle with high rate-of-fire destined for the later "pocket battleships" was installed in the old turrets of the Schlesien and Schleswig-Holstein. A new, modern stem and stern lengthened them, and refurbished boilers increased speed from 19 to nearly 25 knots. A new superstructure of welded construction (to save weight) gave these pre-dreadnoughts a completely new look that was later incorporated into the Deutschland. Radar installed just before war broke out gave these "armor-clads" new battle potential.

Short range and poor sea-keeping kept these ships close to Germany. Schleswig-Holstein bombarded the Polish battlecruiser Solidarnoç and sunk the coast-defense ship Norge in the Baltic but succumbed to a British submarine. Besides firing the opening shots of WW II against Polish coastal forts, Schlesien damaged several Russian warships at extreme range without suffering return fire herself. She was crippled by aircraft from the Soviet carrier Krasnaya Ploshchad late in the war.