In 1858 he visited France, where he minutely investigated the state of the French army, but it was not long before he was recalled, for in 1859, in consequence of the Franco-Austrian War, Prussia mobilized her forces, and Frederick Charles was made a divisional commander in the II. army corps. In this post he was given the liberty of action which had previously been denied to him. About this time (1860) the prince gave a lecture to the officers of his command on the French army and its methods, the substance of which (_Eine militaerische Denkschrift von P.F.K._, Frankfort on Main, 1860) was circulated more widely than the author intended, and in the French translation gave rise to much indignation in France. In 1861 Frederick Charles became general of cavalry. He was then commander of the III. (Brandenburg) army corps. This post he held from 1860 to 1870, except during the campaigns of 1864 and 1866, and in it he displayed his real qualities as a troop leader. His self-imposed task was to raise the military spirit of his troops to the highest possible level, and ten years of his continuous and thorough training brought the III. corps to a pitch of real efficiency which the Guard corps alone, in virtue of its special recruiting powers, slightly surpassed. Prince Frederick Charles' work was tested to the full when von Alvensleben and the III. corps engaged the whole French army on the 16th of August 1870. In 1864 the prince once more fought against the Danes under his old leader "Papa" Wrangel. The Prussian contingent under Frederick Charles formed a corps of the allied army, and half of it was drawn from the III. corps. After the storming of the Dueppel lines the prince succeeded Wrangel in the supreme command, with Lieutenant-General Freiherr von Moltke as his chief of staff. These two great soldiers then planned and brilliantly carried out the capture of the island of Alsen, after which the war came to an end.