Oceania has always been at war with...

[Extrocomp]

What's the point of changing the records every couple of years about which nation Oceania is supposed to be at war with. Every time they do it there's always the risk that it could lead to thoughtcrime. When Winston asks O'Brien if Goldstein's book is true, he replies that the description is but the programme of the proles eventually rebelling if nonsense. So that would indicate that there really is a perpetual war between the 3 super-states and that the nations occasionally have to change alliance to preserve the balance of power. But wouldn't it be easier to keep telling the population that Oceania is still at war with Eurasia even if they're really at war with Eastasia? It's not like the population has any contact with Eurasia or Eastasia. And if we assume that the entire war itself is a deception then this kind of record-changing becomes even more stupid.

So why do they do it?

[bazarov]

Because if it's time of friendship with one state, it's better to tell that you have never been in war with them; so the other one must be you enemy then, because you HAVE to be in war with somebody. Otherwise, some things would loose it's point.

[TomGr]

The answer to this question is that the manipulation of the past is a continual, deliberate exercise in doublethink—one way in which the Party exercises power over the minds of its members. A close reading of the book within the book—The Theory and Practice of Oligarchical Collectivism by Emmanuel Goldstein—makes fairly clear what Orwell was getting at.

[ShakeYourSpear]

It seems to me that they may tell the public who they are at war with (whether they are at war with anyone, or at peace with anyone), simply as another way to make sure everyone is in the "proper" mind.
If a person appears to question how Oceania could be against one of the states, while the day before that they were against the other, then they are not exercising doublethink. If they cannot accept that, then they most likely do not accept other proclamations, or will not accept other ones.

This is why I think the Party tells the public about a great change, to "weed-out" and thought-criminals. It is simply another way of making sure everyone is conforming, and is used in the same way and for the same reason that the Physical Jerks are used.