What motives did japan have in attacking the us naval base at pearl harbor?

One of the most important reasons was for the ban imposed on Japan trade functions by President Roosevelt which gave a great set back to the Japanese economy and their trade and military was affected. 
The Japanese were on their way to emerge as the strongest naval power but America was standing in their way, they had to do something about America's great destroyer ships and naval personnel. Also another reason that Japan attacked America is because the US wanted Japan to withdraw from the north-Indo China. All of these reasons combined led to Japan attacking America's navy in Pearl Harbor.   

Democracy and darwinism would be my answer.