But it is not entirely powerless.

It gains in strength as nations come together and understand one another better.
Thus, it can be seen that the study of languages and the free communication of peoples tend to bring about the supremacy of an opinion opposed to this sort of spoliation.
Unfortunately, it often happens that the nations adjacent to a plundering people are themselves spoilers when opportunity offers, and hence are imbued with the same prejudices.
Then there is only one remedy--time.

It is necessary that nations learn by harsh experience the enormous disadvantage of despoiling each other.
You say there is another restraint--moral influences.

But moral influences have for their object the increase of virtuous actions.

How can they restrain these acts of spoliation when these very acts are raised by public opinion to the level of the highest virtues?
Is there a more potent moral influence than religion?
Has there ever been a religion more favorable to peace or more universally received than Christianity?
And yet what has been witnessed during eighteen centuries?
Men have gone out to battle, not merely in spite of religion, but in the very name of religion.
A conquering nation does not always wage offensive war.