RUTHFULLY YOURS

President Trump provided one clue in his statements about Montenegro, a tiny Eastern European country and the newest NATO member. During a July 18 conversation with Fox News’s Tucker Carlson, he suggested that “aggressive” Montenegrins might start a global conflict. To be fair, Mr. Trump didn’t raise the subject; it was Mr. Carlson who suggested that honoring NATO’s mutual-defense obligation might entangle the U.S. in a fight Americans would rather sit out. But once the subject was raised, Mr. Trump pounced—conjuring the specter of Montenegro dragging the U.S. into World War III.

Where did that come from? There’s no evidence that Mr. Trump had any earlier concern about Montenegro, or even that he knew where it was. Though Montenegro joined NATO shortly after he took office, Mr. Trump’s only direct interaction with the country on record came during a photo session at last year’s NATO summit, when he shoved aside Montenegrin President Dusko Markovic to get a spot in the front row.

There is plenty of evidence, however, that Russia is worried about Montenegro’s accession to NATO. The Kremlin promised unspecified retaliation against NATO in 2015 when the alliance first formally offered membership to Montenegro. Senior Russian officials claim that, in adding the Balkan country to the alliance, NATO members violated their promise not to expand the alliance eastward—an assurance they had given Russia during the presidency of George H.W. Bush.

NATO’s commitment to nonexpansion was never binding; it was meant to apply to East Germany and became moot in 1990 when Germany reunified. But that hasn’t stopped Russian intelligence officers from theorizing—perhaps after a vodka or two—that Montenegro could be the flashpoint for World War III, as its neighbor Serbia was for World War I. That worry may not have been familiar to Mr. Trump, but it certainly was to Mr. Putin, a career intelligence officer. In Helsinki, the Russian president had a motive and an opportunity to raise the Montenegro issue.

Mr. Putin’s actions since the summit also have evinced his renewed aggression toward trans-Atlantic solidarity—on a different front. The day after he met Mr. Trump, Mr. Putin remained in Helsinki to meet one-on-one with the president of Finland, a country long torn between the influence of Russia and Western Europe.

The conflict dates to World War II, when Stalin tried to station Red Army garrisons inside Finland as he had done in the Baltic states of Latvia, Estonia and Lithuania. The Finns were forced to yield but tried to get even by allowing German troops free passage through their territory after Hitler invaded Russia in June 1941. When the war ended, the Finns had to give up some territory and be so cautious not to offend their powerful neighbors that their country’s name has become synonymous with submissive neutrality, as in “Finlandization.”

This might seem like ancient history, but the loss of influence over Finland still matters to the Russians—and particularly to Mr. Putin, who regards the collapse of the Soviet Union as a tragedy. And though Finland has declined to join NATO, it nonetheless is increasingly cooperating with the West on defense, having committed in May to a new strategic partnership with the U.S. and Sweden.

In light of this, it is likely that Mr. Putin used his extended stay in Helsinki to persuade the Finns that Mr. Trump had indicated to him an unwillingness to contest Russia’s ambitions in Europe. It wouldn’t have been a tough case to make: Why would a U.S. president so diffident about protecting his own country’s sovereignty as to laugh off election-related Russian hacking be likely to act boldly in protecting Finland’s sovereignty?

Yes, this is speculation. It is possible that the two-hour meeting between Messrs. Trump and Putin was innocuous, that the conversation revolved around golf and grandchildren, and that what looked like abject exculpation of the Russians really was simply a lingual slip on the way to a forceful accusation against the Russians. But Americans must at least take seriously the possibility that Mr. Trump diminished the U.S. commitment to European defense.

Fortunately, it will be possible to repair that damage through a major recommitment to opposing Russian aggression. The Trump administration’s policies—as opposed to the president’s recent words—already offer a firm basis for such a strategy. Unlike his predecessor, Mr. Trump has sent arms to Ukraine to aid in its resistance to Russia’s attempts to seize its territory. That lethal aid should be sustained and enhanced, specifically to offset the advantage the Russians and their proxies have in armor, artillery and cyber and electronic warfare.

Although President Trump has suggested that joint military exercises are more costly than effective, it would be useful to conduct such exercises more often and with more troops to deter Russia and reassure American allies. The U.S. should also hasten to rebuild its Atlantic Fleet, and especially its rusty antisubmarine-warfare capability, to counter the growing Russian naval presence off America’s eastern coast.

And to retaliate against Russian election interference in a way that would catch the attention of former KGB officer Vladimir Putin, the FBI should consider putting Russian intelligence officers under 24/7 in-person surveillance, as the Russians do routinely to U.S. personnel in Moscow.

Those steps would show our European allies—and Mr. Putin—that America’s actions speak louder than its words.

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