There is nothing more common than to hear of men losing their energy on being raised to a higher position, to which they do not feel themselves equal.

[Nichts gewöhnlicher ist als Beispiele von Männern, die ihre Thätigkeit verlieren, sobald sie zu höheren Stellen gelangen, denen ihre Einsichten nicht mehr gewachsen sind.]

Karl von Clausewitz (1780-1831) Prussian soldier, historian, military theorist

On War [Vom Kriege], Book 1, ch. 3 “On Military Genius [Der Kriegerishe Genius],” (1.3) (1832) [tr. Graham (1873)]
    (Source)

(Source (German)). Alternate translations:

There is nothing more common than to hear of men losing their energy on being raised to a higher position, to which their abilities are no longer equal.
[tr. Jolles (1943)]

No case is more common than that of the officer whose energy declines as he rises in rank and fills positions that are beyond his abilities.
[tr. Howard & Paret (1976)]