The Gleiwitz Incident was a false flag operation carried out by the Nazis on the evening of August 31, 1939. One of several similar incidents taking place around that time, all involving Germany and Poland, the goal was to gain sympathy and to provide a casus belli for a Nazi invasion of Poland, which would start the next day and ultimately lead to World War II.
The incident involved a group of Nazis, dressed as Polish soldiers, that "attacked" a German radio station, Sender Gleiwitz. The "attack" was little more than a short seizure followed by a Polish anti-German radio broadcast; there were no actual casualties from the Nazi side.
In order to make the attack believable, the Gestapo murdered a German farmer who was a known Polish sympathizer, along with several prisoners from the Dachau concentration camp who were drugged, shot at the site and deliberately disfigured, then presented the bodies as "evidence" of a Polish attack on Germany.
International reaction to the incident was one of skepticism, especially as no independent investigation was permitted.