Tough one this. Germany was demeaned and humiliated as a nation by the Treaty of Versailles at the end of the Great War in 1918, so the seeds for a further conflict were sown then.
Herr Hitler had Nationalistic thoughts and actions well before his political defeat of Hindenburg and ascendancy to the Reichstag in 1933, but most historians agree that the declaration of war by the British on 3rd September, 1939 signalled the totality of the future conflict. Until then there had been little effective opposition to Hitler's rise and land grabbing activities.
The British had not prepared well for the conflict and the German resources were superior, and for a while nothing much seemed to happen (phoney war) but the British threw everything into the conflict. The USA joined after the Japanese bombs hit Pearl Harbour, Hawaii, on 7th December, 1941 and the conflict was then truly global.