The 1865 trial of Henry Wirz (right) before a military commission is often touted as a prominent early war crimes prosecution. Wirz, Swiss by birth, was a Confederate Captain. After spending a year in Europe as a special emissary to Jefferson Davis, the president of the Confederacy, he was appointed commander of Camp Sumter (below) near Andersonville, GA. The prison housed Union POWs. At one point, its population swelled to 33,000 persons, making the prison the 5th largest city in the Confederacy, according to one report. Of the almost 50,000 prisoners detained in the camp during the war, over 10,000 apparently died of disease and malnutrition given the over-crowded and squalid conditions.
At the end of the War, General Robert Lee and other high level 
Confederate officials (including the Confederate Secretary of War) were to be charged with the broad crime of conspiring to injure the health of Union soldiers held in the Confederate states. However, in August 1865, President Andrew Johnson (left) ordered that the high-level charges be dropped. Nonetheless, Wirz was eventually prosecuted for mistreating and murdering Union soldiers detained in the prison in violation of the laws and customs of war. Several former prisoners testified against him.
Confederate officials (including the Confederate Secretary of War) were to be charged with the broad crime of conspiring to injure the health of Union soldiers held in the Confederate states. However, in August 1865, President Andrew Johnson (left) ordered that the high-level charges be dropped. Nonetheless, Wirz was eventually prosecuted for mistreating and murdering Union soldiers detained in the prison in violation of the laws and customs of war. Several former prisoners testified against him.In his defense, Wirz argued that under the circumstances of the war, he was unable to ensure proper conditions in the prison and was otherwise just following orders. In pleading his case, he wrote:
I do not think that I ought to be held responsible for the shortness of rations, for the overcrowded state of the prison (which was in itself a prolific cause of the fearful mortality), for the inadequate supplies of clothing, and of shelters &c. Still I now bear the odium, and men who were prisoners here seemed disposed to wreak their vengeance upon me for what they have suffered, who was only the medium, or I may better say, the tool in the hands of my superiors.
May 7, 1865 letter from Capt. Hy Wirz to Maj. Gen. J. H. Wilson. (This letter, the indictment, and judgment against Wirz are 
available in U.S. Army, 8 The War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies.)
available in U.S. Army, 8 The War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies.)The military commission rejected Wirz’s defense and sentenced him to death by hanging. Notwithstanding that many wrote to President Andrew Johnson pleading Wirz’s pardon or at least the commutation of the death sentence, Wirz was hanged on November 10, 1865 (left). On the gallows, he reputedly stated:
I know what orders are. And I am being hanged for obeying them.
