How does George Orwell use language and rhetorical devices to develop the themes and achieve specific effects in 1984?
The entire structure of Oceania relies on Newspeak, and the control of language by the government. For example, the 3 slogans play off people's attraction to simple, easy-to-remember catch phrases. Their parallelism embeds them in the mind and ensures they will be repeated and believed. Controlling language means one controls the population. If you can manipulate how people communicate, you can manipulate their actions and beliefs. This is why freedom of speech and press is so important...otherwise, people would really believe that "War is Peace", "Freedom is Slavery", and Ignorance is Strength".
Newspeak is a language created by stripping English words of their original meaning or eliminating them entirely. This narrows the range of ideas that can be expressed, so the number of words continually shrinks. Word forms and grammar are simplified, as is pronunciation, so that eventually the number of readers can be kept to a minimum. Thus, when Newspeak appeared citizens were unable to read about the past, or express ideas that were counter to what the Party wanted them to think.