He is a barbarian, and thinks that the customs of his tribe and island are the laws of nature.

From Caesar and Cleopatra (1899) by George Bernard Shaw.

CLEOPATRA [interrupting him] Are you not going to speak to me?

CAESAR. Be quiet. Open your mouth again before I give you leave, and you shall be eaten.

CLEOPATRA. I am not afraid. A queen must not be afraid. Eat my husband there, if you like: he is afraid.

CAESAR [starting] Your husband! What do you mean?

CLEOPATRA [pointing to Ptolemy] That little thing.

[The two Romans and the Briton stare at one another in amazement.]

THEODOTUS. Caesar: you are a stranger here, and not conversant with our laws. The kings and queens of Egypt may not marry except with their own royal blood. Ptolemy and Cleopatra are born king and consort just as they are born brother and sister.

BRITANNUS [shocked] Caesar: this is not proper.

THEODOTUS [outraged] How!

CAESAR [recovering his self-possession] Pardon him, Theodotus: he is a barbarian, and thinks that the customs of his tribe and island are the laws of nature.

BRITANNUS. On the contrary, Caesar, it is these Egyptians who are barbarians; and you do wrong to encourage them. I say it is a scandal.

CAESAR. Scandal or not, my friend, it opens the gate of peace. [He addresses Pothinus seriously]. Pothinus: hear what I propose.

The moral trade-offs in pursuit of peace have been around in history and literature for a long time.