In a separate thread, Jeanette gave a historical example of such mass evacuations, and it seems that she has in mind, among other things, large-scale persecution of certain groups. This sort of stuff is really hard to gauge -- when is it so bad that you can no longer get out, and when is it not so bad that you can still stay? I'd wager the window is quite narrow. Six million people misread the situation in WW2. How can we do better?