The major world event marked by the date September 1, 1939, is the start of World War II. On this date, Germany, led by Adolf Hitler, invaded Poland, prompting Britain and France to declare war on Germany shortly afterward. This invasion marked a significant escalation in tensions that had been building across Europe due to aggressive expansionist policies of totalitarian regimes. The war would ultimately involve many nations and lead to unprecedented global conflict, resulting in millions of casualties and significant geopolitical changes.