WWII was triggered by Germany’s invasion of Poland, on 1 September 1939.
This invasion started as a local war, not a world war. It became a world war on 3 September 1939, when Neville Chamberlain declared, in the name of UK and France, war on Germany – when the two European empires declared war on a revisionist Germany.
On 17 September 1939, when following Ribbentrop-Molotov secret protocol, Soviet Union invaded Poland, UK and France did not declare war on the Soviet Empire.
In November 1939 Soviet Union annexed all Polish occupied terrirory, the secret Hitler-Stalin agreements being still valid in parts of Eastern Europe (Poland, Romania, Slovakia).