The Union army issues General Orders No. 100, which provided a code of conduct for Federal soldiers and officers when dealing with Confederate prisoners and civilians. The code was borrowed by many European nations, and its influence can be seen on the Geneva Convention.
The orders were the brainchild of Francis Lieber, a Prussian immigrant whose three sons had served during the Civil War. One son was mortally wounded while fighting for the Confederacy at the Battle of Williamsburg, Virginia, in 1862. Lieber's other two sons fought for the Union. Lieber was a scholar of international law who took a keen interest in the treatment of combatants and civilians. He wrote many essays and newspaper articles on the subject early in the war, and he advised General Henry Halleck, general-in-chief of the Union armies, on how to treat guerilla fighters captured by Federal forces.
Halleck appointed a committee of four generals and Lieber to draft rules of combat for the Civil War. The final document consisted of 157 articles written almost entirely by Lieber. The orders established policies for, among other things, the treatment of prisoners, exchanges, and flags of truce. There was no document like it in the world at the time, and other countries soon adopted the code. It became the standard for international military law, and the Germans adopted it by 1870. Lieber's concepts are still very influential today.