Good rare stamp.

The formal beginning of World War II was marked by the Nazi German invasion of Poland on 1 September 1939, followed by the Soviet invasion of Poland on 17 September. On 28 September 1939 Warsaw capitulated. As agreed earlier in the Molotov–Ribbentrop Pact, Poland was split into two zones, one occupied by Nazi Germany, the other, including all of Kresy, fell under the control of the Soviet Union. In 1939–41, the Soviets deported hundreds of thousands of Poles to distant parts of the Soviet Union. The Soviet NKVD secretly executed thousands of Polish prisoners of war (inter alia Katyn massacre) ahead of the Operation Barbarossa. German planners had in November 1939 called for "the complete destruction" of all Poles and their fate, as well as many other Slavs, was outlined in genocidal Generalplan Ost. The Defense of the Polish Post Office in Danzig (Gdańsk) was one of the first acts of World War II in Europe, as part of the Invasion of Poland. On September 1, 1939, Polish personnel defended the building for some 15 hours against assaults by the SS Heimwehr Danzig (SS Danzig Home Defense), local SA formations and special units of Danzig police. All but four of the defenders, who were able to escape from the building during the surrender, were sentenced to death by a German court martial as illegal combatants on October 5, 1939 and executed.