The one where an economic crisis and two bad election cycles is enough to get people to pull out the swastikas and fasces.
Liberal Democracies are inherently fragile* and prone to authoritarianism when faced with any kind of substantial social pressure. Make that pressure severe enough then they will have fascism develop within their societies and institutions.
It's possible to build a lasting society built on the representative forms that have dominated the Anglo-European world, but the attachment they have to the economic institutions of their societies are an invitation to instability. Instability gives cause to weaken those instititutions (economic, political, or anything else; nothing in society is truly sacred once a society's gone fascist): civil liberties will suffer, social frustrations find their outlet in one way or another and on the whole the society becomes more toxic and volatile by virtue of an everpresent fixture of its structure. You know how the story goes.
Political dissidents end up in jail, or dead.
Undesirables end up in jail, or dead.
And the political economy loses spontaneity and flexibility as more and more space is taken up by whatever it is that's become the new center of power.*
So long as these forms of government/social organization take the forms that they do, they will be vulnerable to fascism because they will always have the ingredients for its formation.
*The "inherence" is that
Liberal Democracies are inextricable from capitalism, and even if the political and social institutions of any country are deep rooted and healthy, there is always a point of weakness in the economic standing of the nation. Capitalism creates economic crises, always has. Some are worse, some are better, but they're a part of the system and the system is liable to suffer for it at one point or another.
*That last bit can be the "strongman dictator", it can a party itself, it can be a religious organization, it can be some cabal of collaborating capitalists, doesn't really matter. Fascisms tend to share aesthetic more than structure.