The event that marked the beginning of World War II in Europe was the invasion of Poland by Germany on September 1, 1939. This aggressive act prompted Britain and France to declare war on Germany days later. The invasion utilized blitzkrieg tactics, a form of warfare characterized by swift, surprise attacks combining air power and fast-moving ground troops. The fall of Poland demonstrated the effectiveness of these tactics and marked the start of a broader conflict that would engulf most of Europe and eventually the world. This invasion is often cited as the catalyst that set off the global conflict leading to World War II.