How did Germany's invasion of Poland affect the start of World War II?

Germany's invasion of Poland on September 1, 1939, is widely recognized as the official beginning of World War II. This aggressive action violated treaties and agreements made after World War I, particularly the Treaty of Versailles, which aimed to contain German expansionism. By invading Poland, Germany not only sought to regain previously lost territories but also aimed to test the resolve of France and the United Kingdom, who had pledged to protect Poland.

In response to this invasion, both France and the United Kingdom declared war on Germany on September 3, 1939. This declaration ended the period of relative peace in Europe following World War I and marked the escalation of conflict into a full-scale war that would eventually engulf many nations around the globe. The invasion thus signified a pivotal shift in the geopolitical landscape, setting the stage for the devastating global conflict that followed.