What did R' Aharon Kotler advise talmidim to do before WWII?
Here is a description of a letter that R' Aharon Kotler sent to R' Gedalya Schorr after R' Aharon found out that R' Schorr was going back to America. This letter was written in the summer of 1939. (source Hamodia magazine Parshas Chukas))
...he could calmly remain in Kletzk and that he did not have to worry about a war in the near future
WWII broke out less then 2 months later and if R' Schorr had followed R' Aharon's advice he most probably would not have survived.
The question we need to ask is what is the lesson we need to take from here? The answer IMHO is that no one is infallible. Yes, RAK was a great talmid chacham but he (and almost all of the Gedolim in pre-war Europe) completely misread the situation before WWII.
RYBS said that R' Chaim thought that there would be Jews living in Brisk in 1979 just like they lived there in 1879 and therefore he didn't see the need for changes like secular education etc.
Today, the world is changing so fast that חדש אסור מן התורה doesn't work.