Sept 1, 1939. After securing a non-agression pact with the USSR (that secretly allowed for the partition of Poland by the Soviet Union and Germany) on Aug 23, Germany invaded Poland without a declaration of war at 4: 45 AM. Two days later, Britain and France declared war, with Canada, Australia, New Zealand and South Africa soon following with their own declarations. Poland, overwhelmed by German air and land power, was in German and Soviet hands before the month concluded.
Image: Public Domain, photographer unknown, via Wikipedia.org.