Over this Memorial Day weekend, while the chief magistrate of our venerable republic was celebrating himself in a tweet instead of the sacrifices of our veterans, I thought of the words of Joseph Welch to U.S. Sen. Joe McCarthy, a disciple of the same Roy Cohn who taught the con man in the White House the black arts of character assassination: “Have you no sense of decency, sir, at long last? Have you left no sense of decency?” This is what known as a rhetorical question. In the case of Donald J. Trump, it is painfully clear that he never had one to begin with.

So instead of dwelling on the president’s tweets, my thoughts, like his, it seems, turned to Bob Mueller. We had some dealings over the years, and they may be instructive for those like me who are inclined these days to despair.

It will be 30 years ago this Dec. 21 that Pan Am Flight 103 exploded in the skies over the Scottish village of Lockerbie, killing all 259 of the passengers and crew as well as 11 persons on the ground. If the explosion had taken place over the North Atlantic, it would have been impossible to recover the debris. Instead, investigators scoured hundreds of miles of the Scottish countryside, and they were able to recover almost the entire aircraft, reassembling it like a great jigsaw puzzle, some 10,000 pieces.

The reconstruction of the fuselage allowed the explosion to be traced to the spot where a suitcase had been placed containing plastic explosives with an explosive trigger supplied to Libya by a Swiss company. A witness was found who could testify that the clothing in that suitcase had been bought in Malta by Libyan agents, and that the suitcase had been placed on a conveyor belt at the airport in Valletta using the special access the Libya agents had to the baggage area, shipped to Frankfurt and loaded on board the Pan Am aircraft.

The investigation was supervised for the Justice Department by an assistant attorney general at the Criminal Division named Robert Swan Mueller III – “Bobby Three Sticks,” he was sometimes called, though presumably not to his face. There were various conspiracy theories and suspicions that Syria had been involved, but the evidence of Libyan responsibility was conclusive. It took almost three years before the Justice Department and the FBI had enough evidence to present indictments of Libyan agents to a grand jury.

Now it may seem implausible, but in those distant days the United States was equipped with a working foreign ministry – we called it the State Department – and I was in its counterterrorism office. Criminal investigations by the Justice Department are not normally coordinated with the State Department, to put it mildly, but in this case Assistant Attorney General Mueller made an exception. In October 1991, some two weeks before the indictments were to be handed down, he came to us and said in effect: “A grand jury is about to indict two Libyan agents for the bombing; I will give you two weeks before this is public to decide what to do about it.”

After some internal debate, President George H.W. Bush decided to invoke the force of international law against Libya rather than bombing the country, with the result that the Libyans abandoned international terrorism once and for all and eventually paid billions of dollars in compensation to the families of the victims. This outcome would not have been possible without the extraordinary investigation supervised by Bob Mueller, obviously – but it was also made possible by his decision to give the State Department the time to devise and carry out a successful political strategy. Most prosecutors in his place would not, I think, have done that.

Many years later, after the assassination in Benghazi of our ambassador and three of his colleagues on Sept. 11, 2012, I came out of retirement to run our shattered embassy in Tripoli.

An FBI team was there investigating the Benghazi attacks, and I spoke with then-FBI Director Mueller from time to time on the secure phone. Before going out to Tripoli I had called on him at FBI headquarters, and I made a small joke about having made the mistake of volunteering for the job. “Oh, no,” he said, as serious as the grave, “you must always volunteer.” And he meant it.

President Trump spent part of the Memorial Day weekend tweeting out lies about the Mueller investigation, and he seems to be increasingly worried about it. Based on my modest experience with the man in question, he should be.