However highly we must value courage and steadfastness in war, and however little prospect of victory there is for him who cannot resolve to seek it by the exertion of all his strength, still there is a point beyond which perseverance can only be called desperate folly, and therefore cannot be approved by any critic.

[Wie hoch auch der Wert des Mutes und der Standhaftigkeit im Kriege angeschlagen werden muß, und wie wenig Aussicht der zum Siege hat, der sich nicht entschließen kann, ihn mit der ganzen Kraftanstrengung zu suchen, so gibt es doch einen Punkt, über den hinaus das Verharren nur eine verzweiflungsvolle Torheit genannt und also von keiner Kritik gebilligt werden kann.]

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

On War [Vom Kriege], Book 4, ch. 9 “The Battle: Its Decision [Die Hauptschlacht. Ihre Entscheidung],” (4.9) (1832) [tr. Jolles (1943)]
    (Source)

(Source (German)). Alternate translations:

However highly we must esteem courage and firmness in war, and however little prospect there is of victory to him who cannot resolve to seek it by the exertion of all his power, still there is a point beyond which perseverance can only be termed desperate folly, and therefore can meet with no approbation from any critic.
[tr. Graham (1873)]

No matter how highly rated the qualities of courage and steadfastness may be in war, no matter how small the chance of victory may be for the leader who hesitates to go for it with all the power at his disposal, there is a point beyond which persistence becomes desperate folly, and can therefore never be condoned.
[tr. Howard & Paret (1976)]