When the shooting started at Fort Sumter on April 12, 1861, Lincoln became a war president barely a month into his first term in office. As a novice commander in chief, his inclination was to deny that the international laws of war had any relevance to the South's war of rebellion. The rebels were criminals, he insisted, not soldiers. Members of Congress and European statesmen pressed him to take international law more seriously. But Lincoln dismissed "the law of nations," as international law was then called, as a curiosity that country lawyers like him knew little about.
But that would change. The interesting story is here. An earlier related essay is also in Salon: Ye Olde Gitmo: When Americans were unlawful combatants.