What is the main idea of "RAN" directed by Akira Kurosawa?

Yesterday,I watched this film that was directed by the famous Akira Kurosawa...
It is very long ..It has grand scenes..
But I just can not catch its main idea,and I still cannot understand those Japan-style phylosophy in this film...
Why the Lord Hidetora Ichimonji changes from a man of hero to a cowardly guy..Why he cannot realize which son is really good and the one he chose is not a good man.....
I know this film is changed from William Shakespeare's tragedy,but I think it's full
of japanese-style phylosophy.....

:genji::halloween

If i recall correctly, Ran is Shakespeare's King Lear in a Japanese setting. Looking up info on the play (as well as reviews of the film itself) might prove englightening.

Here is what wikipedia has to say about it:

Ran (film) - Wikipedia