The trick was that the tweet’s message was implicit rather than explicit. And that is why Twitter did not remove the tweet, as it certainly could do. The company’s weak response shows how utterly incapable it is in dealing with these thorny issues.

It so happens that in recent weeks, including at a fancy-pants Washington dinner party this past weekend, I have been testing my companions with a hypothetical scenario. My premise has been to ask what Twitter management should do if Mr. Trump loses the 2020 election and tweets inaccurately the next day that there had been widespread fraud and, moreover, that people should rise up in armed insurrection to keep him in office.

Most people I have posed this question to have had the same response: Throw Mr. Trump off Twitter for inciting violence. A few have said he should be only temporarily suspended to quell any unrest. Very few said he should be allowed to continue to use the service without repercussions if he was no longer the president. One high-level government official asked me what I would do. My answer: I would never have let it get this bad to begin with.

Now my hypothetical game has come much closer to reality. In using a quote to hide behind what he was actually trying to say, Mr. Trump was testing the system, using a tactic that is enormously dangerous.

It’s important to stress that what Mr. Trump is doing is no different from what various autocrats and haters around the world are doing with social media platforms to push their malevolent agendas. With this latest move by the troller in chief, with no reaction from Twitter, it’s official that the medium has been hijacked by those who want to take advantage of its porous and sloppy rules.

To be fair, I doubt the Twitter chief executive and founder Jack Dorsey could have imagined that a United States president would be so shameless and willing to cause harm by using the digital tools he built.

Well, Mr. Trump has — over and over again. And so it is incumbent on the giant social platforms to prevent the president from dangerously weaponizing their tools. If not, when the history of this era is written, they will be judged as completely failing in their duties as citizens.