World War 2, 1939-1945, has a number of causes. Some historians argue that it was a mere continuation of the First World War, and that the causes of that war should be looked at also. However, the real answers can be found in the period 1918-1939.
After the first war, the peace settlement of Versailles was drafted and in many respects imposed upon Germany and their allies. This treaty, provided it was adhered to, limited the German military to virtually nil, demanded reparation payments and annexed land from Germany. It left the German nation, defeated and in financial difficulties, somewhat disappointed. The formation of the Nazi party had at its heart the overturning of this treaty. Plus further territorial ambitions which would later develop.
The Nazis at first attempted a failed putsch in 1923. Thereafter they campaigned within the Republic's elections. They did not fare especially well in these up to a point. The Wall Street Crash in 1929 left Germany in a most difficult economic position. The series of coalition governments within the Wiemar Republic had not been able to alter this course and as such the Nazis grew in popularity. Then, in 1933, they won enough seats to form a coalition government.
An extreme party had won a small sample of a coalition government. This did not deter the opposition who felt that that was all they would get. Contrary to expectations, via dubious means, they actually consolidated and a dictatorship was formed. Likewise, in Asia, a military dictatorship was established in Japan.
After consolidating internally, the Versailles settlement began to be challenged by the Nazis. Britain and France pursued a policy of appeasement that would aim to keep the peace in Europe. Alas this policy, despite its good intentions, did not obtain these goals. It allowed for the German military occupation of the Rhineland, and the rebuilding of German armed forces generally. It gradually lessened reparation demands, and also allowed for the Saar Plebiscite and a reunion with Austria. Had the French been firmer in upholding the treaty at this point, say for example in protecting the Rhineland, then the German military could have been easily defeated. Likewise, the territorial demands allowed for German expansion into Czechoslovakia and the Polish corridor. By 1939 the Nazis had almost wholly achieved their objective of overturning Versailles.
So it must be said, that one cannot blame the Versailles treaty for this war; for it had wholly been overturned by 1939, without the need for German military action. By annexing the rest of Czechoslovakia, the Munich agreement was violated by Germany. The nation of Poland was a product of the treaty of Versailles, and as such the invasion of Poland was linked in that respect. But Britain and France promised that they would protect this nation. As such the invasion of Poland was an act of war with the French and British also. Likewise, invading Russia had nothing to do with the Treaty of Versailles either.
Germany's re-militarization, helped by the compromising British and French, was also aided by the economic recovery between 1933-1936. The depression had had its effect, and thanks to policies like public work schemes began to ease. 1936 was a crossroads for the Nazis. Credited with economic recovery, they chose to continue rearmament. The economic's minister, Hjalmar Schacht, was skeptical arguing: "you cannot have both guns and butter." This was ignored, and Schacht was sidelined. Significant, for it indicated that ultimately the Nazis were planning for a war of some description in Europe. That war, the invasion of Poland, ultimately led to a more general war in Europe between France and Britain against Germany for a second time.
In Asia, the Japanese were also engaging their military by invading parts of China in 1939. Their military dictatorship ensured Japanese aggression in the pacific, culminating in the bombing of Pearl Harbor in 1941. The pacific war begun in earnest, caused by the hefty goals of Japanese expansion.
American isolationism had also ended. That policy, like appeasement, came at a price for the Americans too. The League of Nations, founded in 1918, was weakened by the policy of American isolationism. The Americans chose not to be a part of an international body they were instrumental in creating. A beacon of peace that this organization was, without the US it lacked the teeth to really keep the peace.
With the benefit of hindsight, the policy of appeasement was the wrong policy to pursue. Equally it highlighted the wish for peace by Britain and France. The political extremists within Nazi Germany, however, evidently took this for granted and ultimately the continued enlargement of the German military allowed for the invasion of Poland and the USSR later. The Wall Street crash of 1929, undoubtedly was a factor that assisted in the rise of political extremist groups of which the Nazis were one. Had this not happened then perhaps the Wiemar Republic would have been more stable in 1933, and the Fascists been subdued within Germany and possibly even Japan. Political moderation might still have won the day, and Germany remained a democracy. However it did not, and there is no question that the extremist parties that emerged from the 1930s in Germany and Japan led to the violation of national borders in Europe and Asia which caused World War 2.