On 1st September, 1939, Nazi Germany invaded Poland from the west, essentially starting World War II.
The Polish army was outgunned, outmanned and at a serious disadvantage from the start. The lasting image is of Polish cavalry charging German Panzers whilst wielding swords. Once the Soviet army invaded from the east, Poland quickly fell and was divided between the two nations.
Following the invasion, on September 3rd both France and Britain declared war on Germany.
Free Poles under the Polish Armed Forces in the West fought on the side of the Allies after the fall of Poland. Polish pilots joined the Royal Air Force and fought in the Battle of Britain. The Poles were the largest non-native contingent of the RAF, with one of their squadrons claiming the highest number of Allied kills in the Battle.
Image: By Ministerstwo Wojny 1938 [Public domain or Public domain], via Wikimedia Commons