With the fall of Poland, thousands of POWs were taken by the German Army, and millions more before the war was over. The question of what to do with those POWs would lead to some of the worst atrocities of the war.
Large numbers of Allied POWs were taken as the Nazis stormed across Europe. Hundreds of thousands of French soldiers were taken to Germany and put into forced labor camps. By 1945 the majority of forced laborers in Germany were French. Two hundred thousand British and Dominion troops were captured in France, Greece, Crete and North Africa. Thousands of Americans were shot down over Germany or taken in combat in Italy. They were all sent to camps in Germany.
When Germany invaded the Soviet Union, the huge encirclements of 1941 swallowed whole Russian army groups. Some five million Soviets went into captivity, including large numbers of Russian women. Only one out five would come home again.
Unlike the indigenous civilian populations that were sent to concentration camps, the POWs were held separately in different camps unless they were found guilty of some war crime and were sent to a death camp. For the Western POWs, that meant they had a better chance at survival then either the concentration camp prisoners or their comrades in Japanese hands. Around 17% of Western Allied soldiers died while in German detention; Allied soldiers in Japanese hands had less than a 50% chance of survival.
Different branches of the German armed forces ran different camps. The RAF and USAAF fliers shot down over Germany were taken to camps run by the Luftwaffe after interrogation. Ground troops were taken to camps run by the Wehrmacht.
As the war progressed against Germany, Hitler issued a series of orders that led to many massacres of western Allied troops. The Commando Order, issued after Dieppe on October 18, 1942, called Commandos bandits and that they should be shot on sight. This led to the execution of Americans dropped into Czechoslovakia by parachute, Canadians killed by the 2nd SS Panzer Division in Normandy in June 1944, and Norwegians attempting to land by boat. The total number of Allied soldiers killed by the Germans will never be known.
POWs were entitled to escape under international law, but the Germans shot any escapees under the Kugel Erlass (bullet decree) which provided for the immediate execution of any flyers found on the ground. Hitler called the Allied air forces terror flyers and Nazi party members lynched many. 50 RAF airmen who escaped from Silesia in 1944 and quickly recaptured were machine-gunned. Their names were posted as a warning to others.
The recidivist escapees, including famous POWs like Churchill's nephew Giles Romilly and Royal Air Force ace Douglas Bader, were taken to Oflag IV-C at Colditz castle. The camp in Saxony saw many of the most outrageous escape schemes, including manufacturing German uniforms, tunneling, and dropping out of the windows to 100 feet below. The castle was floodlit every night, despite the blackout. When the camp was liberated in 1945 the POWs were building a glider in the chapel roof.
Even though Germany signed the Geneva Convention for the humane treatment of prisoners, the Nazi ideology of racial purity and the thirst for vengeance led to millions of POWs dying in forced labor or shot by German ground forces. France, Russia, Britain and the United States tried the commanders who ordered these deaths in courts set up after the war.