24 March 1865, with the fleet having returned to its demobilised, peacetime status, Jachmann was given command of the Marinestation der Ostsee (Naval Station of the Baltic Sea). Later that year, Jachmann moved the fleet to its new main base at Kiel in the newly annexed Duchy of Schleswig. There, he was responsible for establishing the new base, the land defences for which were established at Hörup Haff and Holtenau Bay under the direction of Generals Helmuth von Moltke, the Chief of the General Staff, and Albrecht von Roon, the Prussian Minister of War. Later that year, Jachmann accompanied Roon aboard Loreley for a survey of the Jade Bay, where the naval base at Wilhelmshaven was planned. Jachmann remained in his post in the Baltic Sea during the Austro-Prussian War, commanding the main squadron of frigates and corvettes. Before the outset of hostilities, he sent the gunboat to Elbe to block the movement of an Austrian brigade that was garrisoned in Holstein. Since the Austrian fleet was occupied with Prussia's ally Italy during the war, the Prussian Navy saw relatively little action; the only major operations were conducted in the North Sea, under the command of KzS Reinhold von Werner. On 17 July 1867, Jachmann attended the fleet review held in Spithead, Britain in honour of the Ottoman Sultan Abdülaziz. On 22 August, he became of the director of the Ministry for the Navy, serving under Roon. Jachmann and Roon were good friends, which led to an effective partnership between the two. Jachmann presented his fleet plan in October that year, as part of Roon's overall military budget. The plan projected a ten-year construction program for sixteen ironclad warships, twenty corvettes, and eight avisos. Jachmann included the large number of corvettes to bring pro-colonialist factions to support the plan, though he was himself ambivalent to colonial aspirations. The Reichstag approved Roon's budget without objection. Jachmann was promoted to Vizeadmiral (VAdm—Vice Admiral) on 22 March 1868; he was the first man to hold that rank in the Prussian Navy. By mid-1869, the ironclads that the navy had begun to acquire under Jachmann's direction had started to enter service, permitting the formation of an ironclad squadron. At the start of the Franco-Prussian War in July 1870, Jachmann left Berlin to take command of the ironclad squadron based in Wilhelmshaven. At the time, Jachmann had under his control the ironclads , , ,