The military conflict initiated by the invasion of Poland in 1939 was World War II. This event marked the beginning of a global conflict that involved many of the world's nations, divided into two opposing military alliances: the Allies and the Axis powers. Germany's invasion, which began on September 1, 1939, was characterized by blitzkrieg tactics, emphasizing speed and surprise to overwhelm the Polish defenses. The invasion prompted Britain and France to declare war on Germany, escalating tensions from earlier territorial ambitions and alliances. World War II would ultimately reshape international relations and lead to significant geopolitical changes following its conclusion in 1945.