When Hitler and his army chiefs asked for a pretext for the invasion of Poland in 1939, Heinrich Himmler, Heydrich, and Heinrich Müller masterminded and carried out a false flag project code-named Operation Himmler. German soldiers dressed in Polish uniforms undertook border skirmishes which deceptively suggested Polish aggression against Germany. The incidents were then used in Nazi propaganda to justify the invasion of Poland, the opening event of World War II.
Source: Wikipedia