The paper attempts to show that Vattel established a duty of sovereigns not to interfere in the internal affairs of other states. Although Vattel did not use the terms 'interference' or 'intervention' in any technical sense of the term, it seems justified to see him as an early proponent of what is called today the principle of non-intervention. This will be evidenced by reviewing how Vattel rejected some of the arguments put forward by previous theorists of just war (Gentili, Grotius) who defended the right of European states to intervene in states of the New World in order to punish gross violations of the law of nature and nations. Arguing that the laws of war ought to be applied in reciprocal manner, Vattel questioned the distinction between 'civilized' and 'barbarian' nations on which these previous theorists relied. For him, the true 'barbarians' were those nations who fought wars without even attempting to publicly justify their behaviour.