The fundamental post-WW2 error is the belief that fascism was defeated in 1945. What we should have learned is that about 20-30% of the population generally think fascism sounds pretty good, and whenever we take our eye off the ball... guess who's back?
And the exceptionalism of countries like UK and USA who think it's other countries only. It's all countries where there has been corrupt government, or where they feel much poorer under their government. The poor are useful idiots for fascists who are generally bankrolled by the super-rich.