The official beginning date of the second world war is September first. This is marked by the invasion of Manchuria by Japan, but this doesn't come out of the blue. Europe and Asia were in a tense atmosphere for several years before this due to the rise to power in Germany of Adolf Hitler, also because of the Spanish Civil War with Franco, the invasion of China by Japan, the annexation of Austria by Germany, the Soviet invasion of Finnish territory and several other minor events that had already started to break Europe-Asia relations.
From the European side this doesn't happen without previous warnings: Germany invaded Czechoslovakia in March that year, the German-British naval agreement and the German-Polish peace agreement were both broken in April, and Germany and Russia sign a non-invasion pact and secretly plan to invade Poland in August.
As you can see, the beginning of the war could have been prevented if someone noticed this pattern, but it is obvioulsy easy to say this with an outcome bias, at the time, it would have been almost impossible to predict the start of this conflict.
