Let the one fight for his flag, and the other for his ideal, and let both of them imagine that they are fighting for their country; the struggle will be colossal.

[Que l’un combatte pour son drapeau, et que l’autre combatte pour son idéal, et qu’ils s’imaginent tous les deux combattre pour la patrie; la lutte sera colossale.]

Victor Hugo (1802–1885) French writer, journalist, human rights activist, politician

Les Misérables, Part 5 “Jean Valjean,” Book 1 “The War Between Four Walls,” ch. 21 (5.1.21) (1862) [tr. Hapgood (1887)]
    (Source)

On soldiers and revolutionaries.(Source (French)). Other translations:

Let the one fight for his flag, and the other for his ideal, and let them both imagine that they are fighting for the country; the strife will be colossal.
[tr. Wilbour (1862)]

Let one fight for his flag and the other combat for his ideal, and let them both imagine that they are contending for their country, and the struggle will be colossal.
[tr. Wraxall (1862)]

Let the one fight for his flag, and the other for his ideal, and let them both imagine they are fighting for the country; the strife will be colossal.
[tr. Wilbour/Fahnestock/MacAfee (1987)]

Let the one fight for his flag and the other for his ideal, and let both imagine they are fighting for their country. The struggle will be tremendous.
[tr. Donougher (2013)]