To guard against fatal conceit, triumphant Roman generals famously were warned during celebrations that “thou art mortal.” Michael Cohen is about to provide a far more cinematic reminder to Donald Trump.

The president is in Vietnam trying to make a historic no-nukes deal with North Korea, but his feet of clay will be the sole subject of Cohen’s three days of congressional testimony.

Lest there be doubt that blood will flow, reports emerged before Cohen began testifying Tuesday that he would accuse Trump of committing crimes since he became president.

What a tease! If you were selling tickets to a hanging, or hoping to lure a big television audience for partisan purposes, you couldn’t ask for a better headline.

We’re also told, naturally by anonymous sources, that Cohen will spill the goods on behind-the-scenes life with Trump. That could mean damaging drama, or something akin to outtakes from “The Apprentice.”

Given the long Cohen-Trump history, we’re likely to get a bit of both, leaving us with a stew where allegations of high crimes and misdemeanors are inseparable from farce.

One safe bet is that, whatever the over-under is on how many times Richard Nixon and Watergate will be mentioned, take the over. Democrats take the position that, when it comes to throwing Nixon mud at Trump, too much is never enough.

Those scoring at home could also count the times the media uses the word “bombshell” to describe Cohen’s testimony. You’ll need a calculator.

Impeachment, of course, is a serious matter, but that doesn’t mean it will be treated seriously. This is Congress, which has rightly earned the approval of just 20 percent of the public.

And while Wednesday’s public hearing will be a spectacle, whether it will measurably change the nation’s political dynamic is far from a sure thing.

For one thing, Cohen’s charge that Trump is guilty of criminal conduct will be anticlimactic because we already know the essence of what he’ll say.

In his guilty plea last year to federal charges, which earned him three years in prison, Cohen said he acted at the direction of “Individual 1” in making hush-money payments to women who said they had sexual relations with Trump.

There is little doubt that Trump is “Individual 1,” or that federal prosecutors in New York also believe the president violated campaign-finance laws, although they have not charged him.

Despite an avalanche of critical press coverage at the time, the episode only temporarily damaged the president in public-opinion polls. Indeed, it is worth noting that some recent polling shows Trump at or over 50 percent approval, among the highest of his presidency.

Beyond the public relations aspect, it’s also important to remember that, whatever Cohen tells Congress, he has already told it to special counsel Robert Mueller. As a cooperating witness, Cohen wouldn’t be allowed to testify if he were still being used by prosecutors. He is scheduled to begin his prison sentence in May, so this is probably his swan song.

Cohen always struck me as a pathetic figure, forever doomed to be on the outside of the Trump limelight, looking in. Yet there is no denying that Trump hired and paid him over many years, mostly for the kind of legal work that more reputable firms wouldn’t touch.

Their relationship was thus mutually beneficial, and Cohen once boasted he would “take a bullet for Trump.” But the bravado faded after Mueller’s team started squeezing him, and Cohen’s memory of Trump’s alleged misdeeds grew sharper when New York feds went through his life and businesses with a fine-tooth comb.

In that context, it is worth recalling that one of the charges Cohen pleaded guilty to was lying to Congress. Count on Republicans, especially the aggressive Jim Jordan of Ohio, to highlight that fact and use it to cast doubt on everything Cohen says.

Dems, led by chairman Elijah Cummings of Maryland, will hail the convicted con man — and now disbarred lawyer — as a chastened truth-teller. They will have the advantage, since they will be in charge of the House hearing, giving the nation a glimpse of the importance of the midterm elections, with the get-Trump crowd now running the show.

None of this is to discount the possibility that Trump will be damaged by the three-day onslaught. Having Cohen lay out his charges on camera could remind many Americans why they disliked the president.

And make no mistake about how the hearing will be played and replayed on television and the internet. Throw in leaks of Cohen’s private testimony and, for the anti-Trump media, this will be the closest thing yet to Armageddon.

There isn’t much Trump can do about that. Even in the unlikely event that he and Kim Jong Un announce a dramatic breakthrough, North Korean nukes will take a backseat to anything and everything Cohen says.

Yet this too shall pass, for Cohen himself is just a placeholder for the Mueller report, which all of Washington thinks is coming soon, maybe next week.

That will be the real Circus Maximus, the event that could make or break the Trump presidency on the questions of collusion with Russia and obstruction of justice.

Next to all that, Michael Cohen will be just a footnote.

From Politico: “In his campaign launch video last week, Bernie Sanders singled out the fossil-fuel industry for criticism, listing it among the special interests he planned to take on. But in the final months of the 2016 campaign, Sanders repeatedly requested and received the use of a carbon-spewing private jet for himself and his traveling staff when he served as a surrogate campaigner for Hillary Clinton.”

Note to idealists: Do as the politician says, not as he does.

The significance of the slapdash plan Gov. Andrew Cuomo and Mayor Bill de Blasio released on the MTA is that their congestion-tax proposal is in trouble. Legislators see the cost of about $3,000 a year as punitive on outer-borough and suburban drivers — and they are right.

Worse, the plan isn’t worth the paper it’s written on. The idea that officials are now going to suddenly control MTA spending despite the current structure and union contracts is a fiction.

The only things you can trust are that the cost of living and working in New York will continue to soar and that the stampede for the exits will grow.

Reader Hollister Sykes notes two trends: the popularity of socialism on the left and the failure of conservatives to defend their economic policies. He writes, “The average citizen does not understand capitalism and I blame US high school education. Mandatory courses for all students should include semesters on free markets and business economics. These courses should teach why markets and profit driven businesses produce the most benefit for both the worker and consumer.”