Cuba, Chile, Argentina. They all had democracies, and then collapsed into dictatorships. I'm sure someone more knowledgeable could also name some African or Middle Eastern countries.
Does that even matter? Democracy does not make you inmune to the rest of the world. Democracy is not only an idea and if you want to claim its robustness you have to make sure it is able to cope, in practice, with internal and external opposition, just like all other forms of government. I'm just pointing out that in many cases it has not.
Oh I totally agree, it almost seems that democracy works best when wealth and resources are abundant. This abundance is often at the cost of other nations, in which democracy probably won't last because of the external pressure.
As another example for "western democracy is not assured to work in the long run, many countries after adopting it have gone back to monarchies or dictatorships". So yeah, Weimar Republic.