Daniel was written to encourage the exiled Jews by revealing God's program for them, both during and after the time of Gentile power in the world.  Prominent above every other theme in the book is God's sovereign control over the affairs of all rulers and nations, and thier final replacement with the True King.  Daniel bridges the entire 70 years of the Babylonian captivity (605-536 BC).  Daniel was God's mouthpiece to the Gential and Jewish world, delcaring God's current and future plans.  What Revelation is to the NT prophetically and apocalyptically, Daniel is to the OT.