"Schleswig-Holstein" steamed to the Free City of Danzig in late August 1939, and was moored off the Polish military depot at Westerplatte on the morning of 1 September. She opened fire on the base in Westerplatte, firing the first shots of World War II. The initial German attack was repulsed, leading to heavy fighting that "Schleswig-Holstein" supported for the next week, and culminating in the Polish garrison's surrender on 7 September. Later that month, "Schlesien" joined her sister in bombarding Polish positions along the coast, including the Hel Fortified Area. The ships returned to training duties after the Polish campaign, and in early 1940, "Schlesien" was used as an icebreaker in the Baltic Sea.