What significant geopolitical change occurred by the end of the 1930s regarding Poland?

By the end of the 1930s, the significant geopolitical change regarding Poland was the invasion and partition by Germany. This event, which occurred on September 1, 1939, marked the beginning of World War II when Germany launched a swift military campaign, known as Blitzkrieg, into Polish territory. This invasion was coordinated with the Soviet Union, which subsequently invaded Poland from the east on September 17, 1939, in accordance with the secret protocols of the Molotov-Ribbentrop Pact signed earlier that year.

The outcome of this invasion was catastrophic for Poland, leading to the occupation of its territory by both Nazi Germany and the Soviet Union, resulting in significant loss of life and territorial integrity. This marked a major shift not only for Poland, but for European geopolitics as well, contributing to the broader conflict of World War II and setting the stage for future events in the region. The circumstances of Poland's fate during this period highlighted the failure of earlier diplomatic agreements aimed at safeguarding its sovereignty, particularly in relation to the guaranteed protections that proved ineffective against aggression from both Germany and the Soviet Union.