The defendant was not required to attend the hearing in the state Supreme Court in Lower Manhattan, but there is no show without Trump, right?

There have now been enough of these court appearances for a routine to be established. In flies the former president in his campaign jet. A secret service-led motorcade transports him to the courthouse, Mr Trump sweeps in and out looking bored but dignified, then it is back to Mar-a-Lago to watch the campaign contributions come in and the sales of memorabilia tick upwards.

This time was different. This time was personal. Mr Trump, the organisation that bears his name, and two of his sons, are accused of misrepresenting the value of his assets to obtain better loan terms and other benefits.

The judge in the case has already ruled that he exaggerated the size of his Trump Tower penthouse and over-valued Mar-a-Lago. What remains now is to fix penalties. Attorney General Letitia James, leading the prosecution (and a Democrat), has in mind a fine of $250 million (£207 million) and a ban on Mr Trump doing business in New York.

It sounds almost ludicrous. Like the proverbial angler describing the size of his catch, the 45th president of the US has found out that size really does matter.

SEE MORE Steven Camley's take on Trump trial 

For a man thought incapable of being embarrassed this trial is deeply embarrassing. Mr Trump is no stranger to legal action. For him, and his lawyer and mentor Roy Cohn, suing or being sued was just part of doing business.

But for Mr Trump to be exposed as not as rich and successful as he claimed, well, that has to hurt. It is the kind of move some loser candidate on The Apprentice might pull, not the boss himself. It leaves him open to the charge he hates: that he was only ever the son of a rich guy who gave him a start in business, and not a self-made genius in his own right.

The fraud case has cut him deep, as was plain from his behaviour inside and outside the courtroom on Monday. Red-faced, scowling, shaking his head as the prosecution set out its case, this was not The Donald of the campaign trail, relishing the fight. This was a bully on the back foot.

Outside in the corridor, flanked by political advisers, he held court, calling the proceedings a sham and a scam. “Just so you know,” he added, “my financial statements are phenomenal.” That material for the late-night shows just writes itself.

Donald J having a strop made for great television, but there was more to it than that. This is just the start of a road that could lead to who knows where. The New York trial is expected to last three months. Mr Trump then faces four trials on multiple charges in different states, plus another civil case in New York. All this while running for president.

It is a crazy notion. The proceedings in New York show how even a relatively straightforward trial, one that is judge-only, can take months to play out. The remaining jury trials with lists of witnesses will be more complex and take much longer.

READ MORE Explained in five minutes, the charges against Trump

How is any of this possible? If convicted at any point can Mr Trump still run? If elected, can he govern?

Helpfully, history provides some precedent in the form of one Eugene V Debs. He ran for president three times, on the last occasion while he was a guest of the Atlanta Federal Penitentiary. Prisoner 9653 had been sent to jail for opposing US involvement in the First World War.

There was nothing in the constitution to bar a convicted person from running, so he did. On election day 1920 it was only being a lifelong socialist that stopped Mr Debs getting any further in his aims, though 915,000 people did vote for him.

So Mr Trump, should he be convicted, can stand. It is the next bits that are tricky, the campaigning bit and the being commander-in-chief bit.

Various prosecutors have warned Mr Trump about commenting on his case. As he showed in New York on Monday he is willing to ignore any such advice. Yet if he should be the Republican candidate, how can he not mention the proceedings against him?

There are all kinds of theories and scenarios being suggested. Should you have a spare few hours it is worth taking a trip down the rabbit hole just to see how bizarre the situation could become.

The best bet is that the courts will have to decide, and we know how well that turned out in 2000.

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Erwin Chemerinsky, a professor in constitutional law at the University of California, Berkeley, told the New York Times: “We’re so far removed from anything that’s ever happened. It’s just guessing.”

One of the known unknowns is how the American public will react to seeing a candidate, or a president, being put through trial after trial. The polls have Biden and Trump still neck and neck. One poll caused much cheer in the Trump camp by putting him 10 points ahead, but that was dismissed as an outlier.

Republican voters are keeping the faith with Mr Trump, despite his legal travails, or perhaps because of them. What if his view, that he is the victim of a witchhunt by a Democrat-controlled justice system, takes hold in the wider community? Where does American society go from there?

While it is hard to see any Democrat feeling sorry for Mr Trump and switching their vote, that is not the point here. Outside and inside America, subjecting a candidate to this number of trials in an election year looks unfair, and that is putting it mildly.

Yet what is the alternative? Proceedings could be placed on hold till after the election, but that adds new, greater complications to the mix. A victorious Mr Trump could, in theory, pardon himself, splitting America down the middle as he does.

A second-term President Biden stepping in to forgive and forget? No chance. No wonder politicians favour impeachment, imperfect though it is.

In New York the world has been given clear sight of the wheels starting to turn in the justice system. America is heading to a destination unknown on a road no-one has travelled before. We can only wish her well.