In his first inaugural address, Abraham Lincoln declared that as president he would "have no lawful right" to interfere with the institution of slavery. Yet less than two years later, he issued a proclamation intended to free all slaves throughout the Confederate states. When critics challenged the constitutional soundness of the act, Lincoln pointed to the international laws and usages of war as the legal basis for his Proclamation, asserting that the Constitution invested the president "with the law of war in time of war." As the Civil War intensified, the Lincoln administration slowly...