Where in George Orwell's novel 1984 is a change of external enemies discussed?

One of the most striking moments in George Orwell’s novel
1984 occurs in Part 2, Chapter 9, when the leaders of Oceania
suddenly announce that Oceania is no longer at war with Eurasia but instead with
Eastasia.  Eurasia, formerly a reviled enemy, is now an ally; Eastasia, formerly an
ally, is now a reviled enemy. Ironically, news of these new relationships comes at the
very climax of a rally, nearly a week long, designed to arouse public hatred of Eurasia.
Just when

the
general hatred of Eurasia had boiled up into such delirium that if the crowd could have
got their hands on the two thousand Eurasian war criminals who were to be publicly
hanged on the last day of the proceedings, they would unquestionably have torn them to
pieces—at just this moment it had been announced that Oceania was not after all at war
with Eurasia. Oceania was at war with Eastasia. Eurasia was an
ally.

Part of the point of
describing this change as so sudden (and yet so routine) is that Orwell depicts war
itself as the ongoing focus of the leaders of Oceania (and the other two states). It
almost doesn’t matter which entity is the enemy; what matters most is that war of some
sort, waged against someone, continue so that the population can be controlled by
propaganda and given work to do to support the war effort and sustain the economy. War
isn’t fought on behalf of deeply held ideals but merely as a means of social control and
to keep The Party in power. All the recent history treating Eurasia as an enemy will now
have to be rewritten, but that rewriting will give the party intellectuals something to
do and will also help ensure that the party maintains its grip on the minds of the
general population.

The word “war” appears frequently
throughout 1984, partly because war is presented as a never-ending,
on-going process.  When one reads the novel, the names “Eastasia” and “Eurasia” often
become difficult to keep straight. Thus, readers themselves begin to feel somewhat as
Orwell’s characters feel: confused about who is the enemy at any given moment, but aware
that some enemy must always exist.