The indictment of 12 Russian military intelligence agents last week, on charges they hacked into Democratic National Committee and other servers during the 2016 campaign, raises questions about the timing of the announcement and the work of the hackers themselves. The news came on the eve of the Trump-Putin summit. Why then?

The president was told of the indictments before he traveled. Yet the plain effect of the announcement was to raise further doubts about the wisdom of the meeting—and perhaps to shape its agenda. Neither is the business of the special counsel or anyone else at the Justice Department. The department has a longstanding policy, not directly applicable here but at least analogous, that candidates should not be charged close to an election, absent urgent need, lest the charges themselves affect the outcome. The general principle would seem to apply: Prosecutors are supposed to consider the impact of their actions on significant events outside the criminal-justice system, and to act with due diffidence.

From a law-enforcement standpoint, there was nothing urgent about these indictments. All 12 defendants are in Russia; none are likely ever to see the inside of a U.S. courtroom.

Alternative strategies were available. In 2008 Russian arms dealer Viktor Bout, known to law enforcement as the “Merchant of Death” and the defendant in a sealed indictment, was lured in a sting by U.S. Drug Enforcement Administration agents to Thailand, where he was seized. The Thais, to their great credit, resisted heavy Russian pressure to release him. Instead they fulfilled their treaty obligations and granted a U.S. extradition request.

It has been argued that the objective of last week’s indictments was not to prosecute the defendants but to “name and shame” them. They were named, and even their military intelligence units disclosed—but shamed? In 2006 Alexander Litvinenko, a Russian defector to the U.K., was poisoned in London with polonium from a Russian nuclear facility. Litvinenko had charged that Vladimir Putin was directly responsible for bombing a Moscow apartment building in 1999, an event used as a pretext for the invasion of Chechnya.

Andrei Lugovoi, implicated in the assassination, fled the U.K. and returned to Russia. Not only did Moscow refuse a British extradition request, but Mr. Putin decorated Mr. Lugovoi for “services to the nation.” Mr. Lugovoi was given a seat in the Russian Parliament in 2007. On that record, the 12 indicted hackers are likelier to be lionized than ostracized.

Recall also that the only basis for appointing a special counsel under applicable regulations was the conflict of interest and special circumstance presented by a Justice Department investigation into possibly unlawful conduct by the president’s campaign. Thus the initial order appointing Robert Mueller directs him to investigate “any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump. ” Thus far, numerous Russians have been charged with crimes related to the campaign, and several “individuals associated with the campaign” have been charged with crimes unrelated to the charges against the Russians or to the Trump campaign. No “links” or “coordination” has been charged or even suggested.

Turning to the crime charged, and assuming that the 12 current Russian defendants are guilty, why did they do what they did, in the way that they did?

Despite the wide-eyed, golly-Mr.-Science tone in much of the news coverage, the indictment doesn’t portray cutting-edge Russian intelligence capabilities. The defendants all are said to be members of GRU, Russia’s main military intelligence unit. It is comprised largely of former special-forces types who are looked down upon by their more sophisticated competitors in the SVR, successor to Mr. Putin’s alma mater, the KGB. Their acts, as portrayed in the indictment, obviously were detected—in exquisite detail—by U.S. intelligence services. GRU’s phishing venture, although widespread, was primitive compared with the SVR’s capabilities.

Why would Mr. Putin, an SVR alumnus, give GRU a mission meant to be highly covert? Was this a serious attempt to swing the election to Donald Trump?

At the time of the hacking, virtually no one gave Mr. Trump any chance of winning. Mr. Putin is a thug, but he is not reckless. It seems unlikely he would place a high-stakes bet on a sure loser. Rather, he likely sought to embarrass the person certain to be the new president, assuring that she took office as damaged goods.

Why leave fingerprints? If the only goal was to inflict damage, the new president would have been not only damaged, but also resentful. Even the person who happily posed with a mislabeled “reset” button in frothier days likely would have turned sour.

The point likely was not merely to inflict damage but also to send a warning. Consider the Justice Department inspector general’s report on the FBI investigation into Hillary Clinton’s use of an unauthorized and vulnerable email server. It found that the bureau had concluded the server could well have been penetrated without detection. Recall also that some of the people hacked by GRU agents were aware of that server and mentioned it in messages they sent, so that the Russians too were aware of it. The SVR certainly was capable of an undetected hack.

There are some 30,000 emails that Mrs. Clinton did not turn over, on the claim that they were personal and involved such trivia as yoga routines and Chelsea’s wedding. If they instead contained damaging information—say, regarding Clinton Foundation fundraising—the new president would have taken office in the shadow of a sword dangling from a string held by the Russians.

As we watch the drama of an investigation into whether the president or those close to him committed crimes to help the Russian government, it seems useful to keep in mind not only the possibilities but also the plausibilities.

Mr. Mukasey served as U.S. attorney general (2007-09) and a U.S. district judge (1988-2006).