The Flash could easily go back in time and free ever prisoner of every WWII concentration camp in an hour and even move them into new homes (that he built 0.00001 seconds ago after a quick breakfast lunch and dinner while carrying the person) and even make Hitler die in a way that looks like suicide.

But all The Flash stories that involve time travel seem to mostly focus on how you're not allowed to change anything, so the only direction that story could go is having The Flash being forced to put the people back into concentration camps, which might not sit well with readers.