The right of making war belongs to nations only as a remedy against injustice. It is the offspring of unhappy necessity. This remedy is so dreadful in its effects, so destructive to mankind, so grievous even to the party who has recourse to it, that unquestionably the law of nature allows of it only in the last extremity - that is to say, when every other expedient proves ineffectual for the maintenance of justice.
Emerich de Vattel (1714-1767), Law of Nations