Russians, he added, “believe that making your enemy weaker makes yourself stronger.”

The Dutch intelligence agency, the AIVD, in a publicly released assessment of Russian activities, agreed that measuring the scale of any state-sponsored Russian interference was extremely difficult, as Moscow’s effort to shape public opinion “takes place in a twilight zone between diplomacy and intelligence.”

But the report also noted that the Netherlands had been targeted as part of a “global campaign to influence policy and perceptions on Russia” and, as part of this effort, Moscow had made use of a “network of contacts built up over the years.”

One such contact is Vladimir Kornilov, a Russian-born historian and political analyst who grew up in eastern Ukraine and now lives in The Hague, where he runs a one-man research outfit called the Center for Eurasian Studies. Emails stolen by a pro-Ukrainian hacking group show how Mr. Kornilov offered information and advice to politicians and others in Moscow during his previous work at a Russian-funded research institute in Kiev.

Before the Dutch referendum last year, Mr. Kornilov campaigned against the Ukraine trade deal, describing himself benignly as “a Ukrainian expat in The Hague” who was “stunned by the seemingly endless stream of lies and propaganda” about Russia and felt obliged to respond.

“After the referendum here, everybody thought this was just a Dutch problem, but now we see it was just the beginning,” Mr. Kornilov said in an interview, denying any financial or other links to the Russian state. “There is a huge crisis in the European Union.”

Nor did he agitate alone. He contacted Mr. Van Bommel as well as Thierry Baudet, the head of a conservative research group, Forum for Democracy, which he has since converted into a political party that takes Russia’s side on a host of issues and is competing for seats in the March election on a platform of hostility to the European Union.

During the referendum campaign, Mr. Baudet posted a Twitter message saying that Ukraine “is not a nation state” and retweeted a false report that Ukrainian soldiers had crucified a 3-year-old Russian-speaking boy in eastern Ukraine. The crucifixion story began with an invented report by Russia’s main state-controlled television channel, which interviewed a supposed Ukrainian witness to the crucifixion who was later identified as a Russian actress.