"Preussischer Adler" was sent to the Mediterranean Sea in September 1863 in company with a pair of gunboats, but shortly after they arrived, they were recalled owing to an increase in tension between Prussia and Denmark that resulted in the Second Schleswig War. While on the way back to Prussia, the three ships rendezvoused with a pair of steam frigates from Prussia's ally Austria. The combined squadron attacked a Danish force enforcing a blockade of the German North Sea ports, resulting in the Battle of Heligoland in May 1864. The battle was tactically inconclusive, but the arrival of the Austrian warships forced the Danes to abandon their blockade. Boiler problems kept "Preussischer Adler" from taking part in subsequent naval operations and she underwent extensive repairs in 1867–1868.