These principles can be traced back at least as far as the largely forgotten eighteenth-century Swiss writer Emer de Vattel, who emphasized that nations should be “free and independent.” Benjamin Franklin sent a copy of Vattel’s 1758 treatise The Law of Nations to the Continental Congress in 1775. A year later Vattel’s work influenced the language and purpose of the Declaration of Independence, which used an argument about “unalienable Rights” to assert that the United States were now “Free and Independent.”
Vattel’s main goal in his discussion of civil wars was to bring them under international law by reframing them as standard international conflicts. This was what the Declaration of Independence accomplished in fact: It said that a civil war within the British Empire was actually an international war between separate countries. Vattel said that a mere “sedition” or “insurrection” escalated to a civil war when the rebels fighting against the sovereign had justice on their side, usually as the result of some train of evils or abuses. For Vattel, there was essentially no distinction between a civil war and an international war, since a civil war already represented the splintering of a single political community into two separate and hostile societies. International law rather than domestic law should govern the conflict, and foreign states should feel free to assist one side or another just as they would in a standard war.
The implications of Vattel’s argument were huge. By his logic, foreign powers could intervene at their own discretion in the internal conflicts of other countries. All they had to do was declare that the conflict rose to the level of a civil war and that one party had right on its side. In 1778, France became the first country to recognize the Declaration of Independence and entered an alliance with the United States, helping ensure that the war would be remembered as a successful revolution and not simply a failed civil war.
More than 80 years later, Vattel would have declared that the American Civil War was also a case of two separate, warring nations in which international law, not domestic law, should apply. The Union general and international lawyer Henry Halleck disagreed, contending that the right of foreign powers to recognize both parties in a civil war as independent states was a recipe for chaos….