In the book, the line first comes about when Oceania makes peace with Eurasia and declares war on Eastasia midway through the book. The protagonist sees people happily ripping down anti-Eurasia signs and replacing them with anti-Eastasia signs. All the while, the speakers blare, "Oceania has always been at war with Eastasia." No one questions this, even though they surely know it isn't true. After all, you wouldn't want the Thought Police thinking you were remembering history incorrectly, would you? --Rimrod, at the link above.
I think Orwell's major goof here is that he evidently thinks the proles actually know it isn't true. Actually, it seems that the proles can't remember. This rather takes the sting away from the irony in the neologism "memory holes" - which Orwell portrayed as places to physically destroy evidence in order to hasten the process of forgetting - as in real life there really are memory holes. As in holes in people's memories.
OK, what brought this on?
Hahahahahaha! Hahahahahaha! (Wheeze.) Hahahahahaha!
Oh, wait, he wasn't joking. Iraq and Afghanistan aren't supposed to ring any bells at all.