War is not merely a political act but a real political instrument, a continuation of political intercourse, a carrying out of the same by other means.

[Der Krieg nicht bloss ein politischer Akt, sondern ein wahres politisches Instrument ist, eine Fortsetzung des politischen Verkhers, ein Durchführen desselben mit andern Mitteln.]

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

On War [Vom Kriege], Book 1, ch. 1 “What is War? [Was ist der Krieg?],” § 24 (1.1.24) (1832) [tr. Jolles (1943)]
    (Source)

(Source (German)). Alternate translations:

War is not merely a political act, but also a real political instrument, a continuation of political commerce, a carrying out of the same by other means.
[tr. Graham (1874)]

War is not merely an act of policy but a true political instrument, a continuation of political intercourse, carried on with other means.
[tr. Howard & Paret (1976)]

There is often confusion between this full sentence and the shorter section heading, which is also quoted:

War is a mere continuation of policy by other means.
[tr. Graham (1874) and Jolles (1943)]

War is merely the continuation of policy by other means.
[tr. Howard & Paret (1976)]

Der Krieg ist eine blosse Fortsetzung der Politik mit andern Mitteln.
[German source]