The event that triggered the start of World War II was the invasion of Poland by Nazi Germany on September 1, 1939. This aggressive act violated international treaties and prompted Britain and France to declare war on Germany two days later on September 3. The invasion was part of Adolf Hitler's expansionist policy and aimed to reclaim lost territories and establish German dominance in Europe. This invasion marked the beginning of a conflict that would eventually engulf multiple nations and lead to unprecedented destruction, loss of life, and geopolitical changes.