On 1 September 1939, Germany invaded Poland. With political agreements in place to come to Poland's aid, Britain and France both declared war on Germany within two days. It was the start of a conflict that would erupt over every continent, see the deaths of tens of millions of people, much of central Europe destroyed, the development of jets, intercontinental missiles and computers as well as the Atomic Bomb. 1941 saw the invasion by Nazi Germany of the Soviet Union, the biggest example yet of Blitzkrieg. The war would widen still further at the very end of the year when aircraft of the Imperial Japanese Navy attacked the US naval base at Pearl Harbor. The year would also see the sustained bombing of Malta, the German Blitz on British cities and the invasion of Greece and Yugoslavia. The third in a series covering the Second World War in the air year by year.