The basic cause of World War II was the fact that the three Axis powers were generally unhappy with the status quo in the world after the end of the First World War.  These three countries pushed to change the international system leading the Allies, eventually, to go to war.

After WWI, Italy, Germany, and Japan (particularly the last two) were unhappy with the international order.  Germany was extremely unhappy with the degree to which it had been punished by the Treaty of Versailles.  Japan felt that it was not being given its due as a major power.  It felt that it was being denied the sort of empire that European countries were allowed to have.  Italy, too, felt that it deserved more than it had.

These three countries, therefore, started to push to rectify things.  Germany took Austria and Czechoslovakia and eventually invaded Poland.  Italy invaded Ethiopia.  Japan invaded China and later entered Indochina after France surrendered to Germany.  Ultimately, of course, it attacked Pearl Harbor in an attempt to prevent the US from interfering with its attempts to take an empire.

The invasion of Poland and the attack on Pearl Harbor were too much of a threat to the status quo.  The Allies reacted to these actions by going to war.