At the start of the Second Schleswig War in 1864, the bulk of the Prussian fleet was concentrated in the Baltic Sea. Prince Adalbert was the senior navy commander, and Jachmann, as the senior-most captain, became the squadron commander, with "Arcona" as his flagship. The much larger Danish fleet blockaded Prussia's main ports, trapping Jachmann's ships—"Arcona" and —in Swinemünde. On 17 March, Jachmann took his two ships and the armed paddle steamer out to challenge the Danish blockade force. In the ensuing Battle of Jasmund off the island of Rügen, Jachmann's squadron was defeated by the more powerful Danish squadron and forced to retreat. This was to be the only major naval battle fought in the Baltic during the war. Nevertheless, he was appointed "Konteradmiral" (Rear Admiral) for his aggressive handling of the fleet at Jasmund. The arrival of a Danish ironclad warship precluded any further attacks on the Danish fleet, as the traditional guns that armed Jachmann's ships were entirely useless against the armored vessel.