The invasion of Poland started World War II. This happened on September 1, 1939. The Allies shouldn’t have been surprised by the invasion. There were several aggressive actions by Germany and the Axis Powers prior to the invasion of Poland.
In 1936, Germany moved its military into the Rhineland even though this was prohibited by the terms of the Versailles Treaty. In 1938, Germany invaded Austria. Later that year, Germany announced it wanted to annex the Sudetenland. This was an area in Czechoslovakia where many Germans lived. The Allies met in Munich and gave this land to Germany in a return for a promise by Germany to take no more land. That action, known as the Munich Pact, failed in 1939 when Germany invaded the rest of Czechoslovakia. Italy also invaded Albania in 1939. A very ominous sign occurred in late August 1939 when Germany and the Soviet Union signed a non-aggression pact. This meant Germany, if war began, would have only a one-front war to fight since the Soviet Union and Germany agreed not to fight each other. About one week later, Germany invaded Poland leading to the start of World War II.