Great Britain declared war on Germany after Germans invaded Poland on September 1, 1939.
German troops invaded Poland on September 1, 1939,. It event marked the beginning of World War II. In reaction to German aggression, Hitler's invasion of Poland, Great Britain and France declared war on Nazi Germany.
German forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident. Slovak military forces advanced along with the Germans in northern Slovakia.