In the novel, "1984", why is their world constantly at war?

Although we think of war as a horrible experience, it can be a useful tool for the state.  If there is a constant threat to the well being of a state's citizens, that anxiety can be used to control the people and focus their attention on doing whatever it takes to keep them safe.  Although it is not the same thing, look at what happened in our country after 9/11.  Our government, for better or for worse, asked us to give up some of our traditional civil rights in return for greater safety.  It's still under discussion

In 1984, it doesn't make any difference which state is fighting against which since all the states use the "war" to control their people, to give them someone to hate, to turn over whatever rights (more to things like razor blades than abstract freedom) they may have in turn for safety, and to turn their energy anywhere else than on themselves and the sorry state they live in.

War as tool.