With a vigorous national debate underway on whether Sweden should enter a military partnership with NATO, officials in Stockholm suddenly encountered an unsettling problem: a flood of distorted and outright false information on social media, confusing public perceptions of the issue.

The claims were alarming: If Sweden, a non-NATO member, signed the deal, the alliance would stockpile secret nuclear weapons on Swedish soil; NATO could attack Russia from Sweden without government approval; NATO soldiers, immune from prosecution, could rape Swedish women without fear of criminal charges.

They were all false, but the disinformation had begun spilling into the traditional news media, and as the defense minister, Peter Hultqvist, traveled the country to promote the pact in speeches and town hall meetings, he was repeatedly grilled about the bogus stories.

“People were not used to it, and they got scared, asking what can be believed, what should be believed?” said Marinette Nyh Radebo, Mr. Hultqvist’s spokeswoman.

As often happens in such cases, Swedish officials were never able to pin down the source of the false reports. But they, numerous analysts and experts in American and European intelligence point to Russia as the prime suspect, noting that preventing NATO expansion is a centerpiece of the foreign policy of President Vladimir V. Putin, who invaded Georgia in 2008 largely to forestall that possibility.

In Crimea, eastern Ukraine and now Syria, Mr. Putin has flaunted a modernized and more muscular military. But he lacks the economic strength and overall might to openly confront NATO, the European Union or the United States. Instead, he has invested heavily in a program of “weaponized” information, using a variety of means to sow doubt and division. The goal is to weaken cohesion among member states, stir discord in their domestic politics and blunt opposition to Russia.

“Moscow views world affairs as a system of special operations, and very sincerely believes that it itself is an object of Western special operations,” said Gleb Pavlovsky, who helped establish the Kremlin’s information machine before 2008. “I am sure that there are a lot of centers, some linked to the state, that are involved in inventing these kinds of fake stories.”

The planting of false stories is nothing new; the Soviet Union devoted considerable resources to that during the ideological battles of the Cold War. Now, though, disinformation is regarded as an important aspect of Russian military doctrine, and it is being directed at political debates in target countries with far greater sophistication and volume than in the past.

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