Answer:
For all the Inner Party is concerned, perpetual war is no different than perpetual peace. War, by being everlasting, is really not making any changes to Oceania's current society. When its existence offers no changes and zero impact, it is as if it doesn't even exist. As Goldstein's book puts it: "by becoming continuous, war has ceased to exist." (Orwell, pg. 207)
In the age of Big Brother, "the war is waged by each ruling group against its own subjects, and the object of the war is not to make or prevent conquests of territory, but to keep the structure of society intact." (Orwell, pg. 207) When the main goal of the Inner Party is to establish an equilibrium of its hierarchical society, all it's after is consistency. Constant war and constant peace, they are essentially the same thing. Hence, the doublethink phrase: War is Peace.