The immediate cause was Germany invading Poland on September 1, 1939, and Britain and France declaring war on Germany on September 3, 1939. Problems arose in Weimar Germany that experienced strong currents of revanchism after the Treaty of Versailles that concluded its defeat in World War I in 1918.
Although a variety of different factors caused World War Two, the main event and the trigged for what started WW2 was Germany’s invasion of Poland in September 1939.
Of course the invasion was preceded by decades of political conflict. The Treaty of Versailles, signed at the end of WW1, placed some very harsh restrictions on Germany, which created a feeling of resentment among the Germans. Due to the financial depression of the 1920’s, Germany simply did not have the funds to pay the reparation fees demanded by the treaty.
