It is known notoriously as "double-think", from the book 1984, which gives a reminder how the Party's massive and enormous campaign of large-scale leads to full-blown psychological manipulation of the entire residents of that country. This Party's mind-brainwashing technique breaks down a person capacity to think independent thoughts and this would lead to people believing anything that the Party had said to them, even if it was lies or deceptions, even possessing information that was contradicting earlier beliefs, like
During the Hate Week rally, the Party switches their allegiance with many countries, so the so-called enemies became their allies and vice versa, and the people accepts this fake lies without a moment hesistation, showing how gullible and naive they are.
In the book, this is known as "doublethink". It's from Goldstein's book within the book in 1984.