Why don't the powers ever mount a true invasion of the others?
The point of war, according to the powers that be in the society of the novel, is to perpetuate fear and need, not to secure peace. When Winston reads Goldstein's Book (Section 2, chapter 9), he learns that war, first of all, serves an important economice purpose, particulary in the labor field. In addition, the war effort keeps the citizens in a standard of living which allows them to be more easily manipulated by the government, or
to use up the products of the machine without raising the general standard of living. (Section 2, chapter 9).
In this way, people could be kept from becoming too comfortable or feeling too safe. As the book remarks, comfort and safety lead to intelligence, poses a direct threat to the superiority of the leadership.
Thus, the Inner Party best survives as it wishes when individuals are kept frightened, uneasy and less intelligent through war.