With former special counsel Robert Mueller’s testimony before Congress on Wednesday promising to be the final postscript to his service in the probe of Russian interference in the 2016 election, we can now begin to evaluate the overall record.


Truth be told - and it’s a difficult truth - Mueller, while no doubt methodical, energetic and wholly impartial, looks also to have been an overly restrained prosecutor who stayed his hand when he could have and should have pressed forward.


Arguably, the only figure ensnared for his conduct rather than as a conduit to a bigger fish in the broad probe was former Trump campaign chairman Paul Manafort. Even that case hit a brick wall when Manafort turned squirrelly and uncooperative, possibly in the hope of a Trump pardon.


The first inkling of Mueller’s hesitation was his decision not to subpoena the president, which we’ve known about for some time. It was frustrating that Mueller, overseeing a criminal investigation centering on intent, would not pursue the president’s testimony, especially given Mueller’s plainly correct assessment that his legal claim on the issue would be upheld. Subpoenaing President Donald Trump seemed an obvious and perfectly justified move that held the promise of breaking the investigation wide open.


That decision now seems of a piece with his overall approach and consistent with a half-dozen or more decisions he made at critical junctures.


Mueller adhered to Marquis of Queensbury rules when the Trump camp - including the president - was applying a no-holds-barred cage fighters’ approach, including vicious and false attacks on the special prosecutor.


Mueller’s gentlemanly restraint appears to have driven other decisions. For example, the report details a voicemail from Trump lawyer John Dowd to counsel for former national security adviser Michael Flynn after Flynn had decided to cooperate with the government. Dowd asks to be kept abreast of what Flynn tells prosecutors, emphasizes Trump’s feeling for Flynn and, in a call with Flynn’s attorneys, ultimately threatens to tell Trump that Flynn was being hostile.


The voicemail fairly screams possible witness tampering and obstruction of justice. The obvious prosecutorial move would have been to interview Dowd about the calls and to ask whether Trump was aware of them. But Mueller refrained on the ground that questioning Dowd could have intruded on attorney-client privilege. Yet if Dowd’s overtures were in fact part of a crime, there would be no privilege.


Then there’s Mueller’s decision to give a pass to Donald Trump Jr. The president’s son appeared to be in Mueller’s crosshairs throughout the probe, on questions growing out of the June 2016 Trump Tower meeting that he took to receive negative information about Hillary Clinton from the Russian government, as well as about the truthfulness of his testimony to Congress. But Mueller declined to pursue either matter, citing in the case of the Trump Tower meeting “the high burden to establish a culpable mental state in a campaign-finance prosecution and the difficulty in establishing the required valuation” of the negative information.


Maybe so. But as in other instances, Mueller seems not to have pushed hard to get testimony from Trump Jr. or others who might speak to his mental state, and though the valuation question might have been open to some debate, common sense suggests that the information was sufficiently valuable to justify the campaign’s keen interest in it.


There are several other examples, including Mueller’s decision to close up shop before seeing the prosecution of Trump confidant Roger Stone, charged in January for lying, obstruction and witness tampering, through to the end and walking away from writer Jerome Corsi after extensive negotiations over a perjury charge.


The bottom line is that having passed on the best chance to establish the president’s criminal intent to obstruct justice, namely, by subpoenaing his testimony, Mueller then stepped back from many of the best secondary opportunities to shore up the case on intent and to pressure knowledgeable subjects to provide truthful information rather than stonewall, which they largely succeeded in doing.


And then the second greatest cause of frustration with Mueller’s approach (after the decision not to subpoena the president): His contorted explanation in the report not to provide a bottom-line characterization of Trump’s behavior as criminal, notwithstanding supplying all the building blocks for that determination.


Here Mueller developed and relied on his own self-imposed “fairness concerns” about charging Trump in a setting in which he could not clear himself though a speedy and public trial. It’s more evidence of Mueller’s genuine rectitude, but there is an irony in Mr. By-the-Book’s gratuitously writing a whole new chapter, and it served to greatly confuse the public while permitting Trump to claim victory.


That, combined with his Delphic formulation that “if we could have exonerated Trump, we would have,” left it to Trump antagonists to try to argue in complex sound bites that Trump had committed obstruction. It also gave Attorney General William Barr a hole big enough to drive a truck through with his (mis)characterization of Mueller’s report.


Mueller’s testimony this week will provide an opportunity to clean up that particular mess - though it’s probably more likely than not that he won’t. All of the other failings of the probe are effectively irremediable.


That means first that the probe has fallen well short of full justice, which is a shame, and that the erstwhile targets will crow that they have been fully exonerated, which is galling.


But there is now a far grimmer problem. The efforts by Russia to disrupt the 2016 election, and the Trump team’s efforts to subvert Mueller’s probe, are matters of national import along the lines of 9/11 or the Kennedy assassination. In those instances we had national commissions to get at the truth. It is awful to contemplate that so many of the facts surrounding what happened in 2016 and after might remain forever the stuff of conjecture by historians rather than testimony by witnesses. Here lies Trump’s greatest triumph and Mueller’s greatest failing.


Harry Litman is a columnist with The Washington Post.