I was thinking more in terms of declared hostilities. IMO, the war really started the day that the British and the French forced the ridiculous Treaty of Versailles on Germany. When that document was signed, Round 2 was guaranteed.
The War in Europe started in earnest on 1st September 1939 when the Nazis invaded Poland.
September 1st - Germany invades Poland.
September 3rd - Britain, France, Australia and New Zealand declare war on Germany.
September 10th - Canada declares war on Germany.