Dictatorships usually come about through a military coup (or at least with the support of the military) or following a victory in a civil war. There is almost always an element of force.
But not always. Hitler, for example, came to power following a democratic election. He then installed a whole apparatus of terror and repression, but the real source of his power was the sense that he could not be resisted, and that everyone would do what they were told, since it was too dangerous to do otherwise.
Are we witnessing a similar scenario unfolding before our eyes in the United States? There we have a leader elected according to the US constitution, but acting increasingly as though he is subject to none of the usual constraints on the arbitrary use of power.
Donald Trump took office as President of the United States with an alarming ignorance of the country he leads and its history - and that included what seems to be a complete misunderstanding of how government works in a democracy and of the US President's role in that government.
His experience as a business tycoon and as a reality TV star seems to have persuaded him that being President means you can do what you like - without regard to any other elements of a democratic government.
So, when the courts declared his ban on certain entrants to be illegal, he was outraged - just as he is when held to account by the media. His response to the media is to lambast them for publishing "fake news". And then, when he had problems getting his healthcare legislation through Congress, he threatened that he would "close down" government.
He seems to think that the US courts, the country's legislature and the media are of no account, and that it is the executive alone - in the person of the President - that should exercise unbridled power.
It is his most recent exercise of presidential power, however, that should really set the alarm bells ringing. His "termination" of James Comey, the FBI Director, adds the nation's domestic intelligence and security services, and the nation's prime federal law enforcement agency to his list of those agencies from which he will brook no interference.
It has hit the headlines because it seems so evidently an attempt to close down an FBI inquiry into the Russian involvement in his election campaign. But, important though that issue is, the "termination" has a much wider significance. It sends the message that no one can afford to cross him.
Much now depends on how the country's leaders respond to this startling exercise of arbitrary power. If they just roll over, a major step towards a dictatorship will have been taken.
A despot does not achieve power because he is able personally to injure, threaten or restrain his opponents. He wields power by controlling the power exercised by others - confident that they will not, and dare not, resist. A dictator cannot operate without henchmen to do his bidding.
It is the conviction that resistance is futile - even in the midst of the trappings of democracy - that is the essence of a dictatorship. If Trump is able to "terminate" a top official - one responsible for the country's security - simply to protect himself and his own reputation, and does so with impunity, the lesson will be quickly learnt.
A new FBI Director, for example, will know that he or she takes office on condition that the President's own personal interests must prevail over all other considerations - and everyone else will learn that lesson too.
A huge burden now falls on those who are meant to be guardians and upholders of democracy in the United States. They cannot just close their eyes and pretend not to see.
A dictatorship established in a functioning democracy may seem unthinkable - just as it would have seemed to many in Germany in the 1930s.
But, as Edmund Burke famously warned, "All that is necessary for the triumph of evil is that good men do nothing".
If American political leaders do nothing, the conditions needed to increase and underpin the power of the despot will be met. The leaders of the Republican Party already bear a heavy responsibility for their role in bringing about a Trump presidency. It is time for them to step up to the plate.
Bryan Gould is a former British MP and Waikato University Vice-Chancellor.