Abusive online harassment is hardly limited to pro-Russian internet trolls. Ukraine and other countries at odds with the Kremlin also have legions of aggressive avengers on social media. But pro-Russian voices have become such a noisy and disruptive presence that both NATO and the European Union have set up special units to combat what they see as a growing threat not only to civil discourse but to the well-being of Europe's democratic order and even to its security. This "information war," said Rastislav Kacer, a veteran diplomat who served as Slovakia's ambassador to Washington and at NATO's headquarters in Brussels, "is just part of a bigger struggle." While not involving bloodshed, he added, it "is equally as dangerous as more conventional hostile action." For Aro, the abuse increased sharply last year when, following up on reports in the opposition Russian news media, she visited St. Petersburg to investigate the workings of a Russian "troll factory." The big office churns out fake news and comment, particularly on Ukraine, and floods websites and social media with denunciations of Russia's critics. In response to her reporting, pro-Russian activists in Helsinki organised a protest outside the headquarters of Yle, accusing it of being a troll factory itself. Only a handful of people showed up.

At the same time, Aro has been peppered with abusive emails, vilified as a drug dealer on social media sites and mocked as a delusional bimbo in a music video posted on YouTube. "There are so many layers of fakery you get lost," said Aro, who was awarded the Finnish Grand Prize for Journalism in March. As Aro's experiences illustrate, Finland, a country at the centre of Russia's concerns about NATO's expansion toward its borders, has emerged as a particularly active front in the information wars. A member of the EU with an 830-mile-long border with Russia, Finland has stayed outside the United States-led military alliance but, unnerved by Russian military actions in Ukraine and its sabre-rattling in the Baltic Sea, has expanded cooperation with NATO and debated whether to apply for full membership. Public opinion is deeply divided, making Finland a prime target for a campaign by Russia. "Their big thing is to keep Finland out of NATO," said Saara Jantunen, a researcher at the Finnish Defense Forces in Helsinki, who last year published a book in Finland entitled Info-War. She said that she, too, had been savaged on social media, sometimes by the same and apparently fake commentators who have hounded Ms. Aro.

"They fill the information space with so much abuse and conspiracy talk that even sane people start to lose their minds," she added. Europe's main response so far has been to try to counter outright lies. In November, the EU launched "Disinformation Review," a weekly compendium of pro-Kremlin distortions and untruths. But facts have been powerless against a torrent of abuse and ridicule targeted at European journalists, researchers and others labelled NATO stooges. Pro-Russian activists insist that they are merely exercising their right to free speech, and that they do not take money or instructions from Moscow. The most abusive messages against Aro were mostly sent anonymously or from accounts set up under fake names on Facebook and other social media.

One of her most vocal critics in Finland, however, has openly declared his identity. He is Johan Backman, a tireless supporter of President Vladimir Putin of Russia who highlights the blurred lines between state-sponsored harassment and the expression of strongly held personal views. Fluent in Russian, Backman now spends much of his time in Moscow, appearing regularly in the Russian news media and at conferences in Russia as "a human rights defender." He also serves as the representative in Northern Europe for the Russian Institute for Strategic Studies, a state-funded research group led by a Soviet-era intelligence officer. Backman, who also represents the Donetsk People's Republic, the breakaway state set up with Russian support in eastern Ukraine, denied targeting Aro as part of any "information war." Rather, he insisted that Russia was itself the victim of a campaign of disinformation and distortion conducted by the West. In a recent interview in Moscow, he said that Aro was part of this campaign and that she had tried to curtail the freedom of speech of Russia's supporters in Finland by labelling them as "Russian trolls." All the same, Backman added, her complaints about being targeted for abuse "have been very beneficial for Russia" because they have made others think twice about criticising Moscow. "She says she is a victim, and nobody wants to be a victim," he said. "This changed the atmosphere in the journalistic community."

Backman said he used his own private means to fund his activities in support of what he described as an "entirely defensive" campaign by Russia to counter Western propaganda. His activities, however, invariably follow Moscow's political and geopolitical script, particularly on NATO, which he regularly denounces as a tool for US military occupation. Aside from NATO, Backman's biggest bugbear of late has been Aro and the "Russo-phobic" tendencies that she, in his view, represents. Just days after Aro made her first appeal in September 2014 for information about Russian trolls, Backman told Russian People's Line, a nationalist Russian website, and other media that she was a "well-known assistant of American and Baltic special services." Around the same time, she received a call late at night on her mobile phone from a number in Ukraine. Nobody spoke, and all she could hear was gunfire. This was followed by text and email messages denouncing her as a "NATO whore" and a message purporting to come from her father — who died 20 years ago — saying he was "watching her." The hardest blow, Aro said, came early this year when a Finnish-language news site, MVLehti.net, which is based in Spain and mostly focuses on vilifying immigrants, dug up and published court records that showed she had been convicted of using illegal amphetamines in 2004. She had been fined 300 euros.

The website's headline: "NATO's information expert Jessikka Aro turned out to be a convicted drug dealer." It also posted photographs of Aro dancing in a slinky outfit at a nightclub in Bangkok. Backman requested and received Aro's old case file from the court shortly before the website published the documents. He denied passing them on to the site. The false claim that Aro was a drug dealer triggered an unusual open letter signed by more than 20 Finnish editors infuriated by what they denounced as the "poisoning of public debate" with "insults, defamation and outright lies." The Finnish police began an investigation into the website for harassment and hate speech. "I don't know if these people are acting on orders from Russia, but they are clearly what Lenin called 'useful idiots,'" said Mika Pettersson, the editor of Finland's national news agency and an organiser of the editors' open letter. "They are playing into Putin's pocket. Nationalist movements in Finland and other European countries want to destabilise the European Union and NATO, and this goes straight into Putin's narrative." Ilja Janitskin, the founder and head of MVLehti, who is based in Barcelona, Spain, said in response to emailed questions that he had no connection with Russia other than his surname. His political views, he said, are closer to those of Donald Trump, not Putin.

He added that he had become interested in Aro only after she accused his website of "distributing Russian propaganda." Like Backman, he denied receiving any money from Russian sources, insisting that his website, which in just 18 months has become one of Finland's most widely read online news sources, finances itself from advertising and donations by readers. Aro acknowledged that she had used amphetamines regularly in her early 20s but dismissed as a "total lie" claims that she had been or is a drug dealer. "They get inside your head, and you start thinking: If I do this, what will the trolls do next?" she said. The New York Times