Which event directly triggered the start of World War II in Europe?

The invasion of Poland by Germany

On September 1, 1939, Nazi Germany, led by Adolf Hitler, invaded Poland, thereby triggering the start of World War II in Europe. This brutal attack, justified by false claims of Polish aggression, took Poland and the international community by surprise. In response, France and the United Kingdom, allies of Poland, declared war on Germany. This event marked the beginning of a conflict that would last six years and become one of the most devastating wars in human history, involving dozens of countries and resulting in the deaths of millions of people. The invasion of Poland also highlighted the failed policy of appeasement and paved the way for total war in Europe.