Reasons that George Orwell wrote "1984" include the totalitarian regimes he witnessed, the Tehran Conference in 1944 and the atmosphere in England during World War II. A letter he wrote in 1944 that describes the thesis of the book is reprinted in "George Orwell: A Life in Letters."
Orwell saw political leaders in Germany, Spain and the Soviet Union who had absolute power, and he realized the danger that this presented. He illustrated that danger with the world in his book, where everyone is monitored and must conform to the views of the elite.
The Tehran Conference was a meeting between Franklin Delano Roosevelt, Winston Churchill and Joseph Stalin. Orwell thought the three could plan to divide the world amongst each other. In "1984," Orwell uses this idea with the three super-states of Oceania, Eurasia and Eastasia. The three sides control the world and are in a never-ending war.
Orwell lived in England with his family during World War II, when acts of violence could occur at any time. This atmosphere is replicated in the book.
In his letter, Orwell mentions that he feels totalitarian regimes are on the rise in many parts of the world. He says that the United States and Great Britain are exceptions, but neither nation has known defeat or suffering. He also brings up several ideas that would later be a part of "1984," including that history could be rewritten by whoever was in charge.