The main theme is that if we are not careful with our language, which is the way we interface with the world of things and the world of thoughts, then we will be at the mercy of almost anyone who uses language against us. This was a major concern for Orwell. The other is that if we do not take the trouble to protect our knowledge of the past, it can be "recreated" to make us believe almost anything. That is the meaning of "Who controls the present controls the past; who controls the past controls the future."
These two things are not separate. Controlling the past is, in part, achieved through controlling the language that we use to describe it. How many of us actually know what has happened during their lifetime, never mind the history of our country. We are told (usually by the opposing party) that everything the other party has (almost) ever done is erroneous, ill advised, or just plain wrong. To use the weapons of mass destruction example, it is my recollection that almost the entire world believed the weapons were there; the way it is told now, Bush made up the whole thing. Who will remember in 20 years what actually happened if it's unclear just 5 years later? This is already being used to criticize Bush (about whom no one will care in 2 years anyway), but is it valid or just a tool?
"Who controls the present, controls the past."