Facebook, which long resisted the notion that its social network was weaponized during the presidential campaign, has spent much of the past year grappling with how to make its platform resistant to misinformation and other forms of fake news. Part of that postmortem was compulsory: on Wednesday, The Washington Post reported that Facebook has told congressional investigators it sold about $100,000 worth of ads to a shady pro-Kremlin Russian propaganda company seeking to target U.S. voters. That evidence could be of critical importance as Justice Department special counsel Robert Mueller continues his probe into Russian interference in the 2016 election, including whether the Kremlin was assisted by any members of Donald Trump’s campaign. Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, has already said that he will be seeking further details from Facebook about how the ads were geographically targeted, and whether the Kremlin received any help from people within the U.S.

Those are key questions for Mueller, too, as he investigates alleged collusion between the Trump campaign and Moscow. Experts have suggested that it would be difficult for Russia to have successfully executed its misinformation campaign in the U.S. without granular knowledge of voter data or assistance with demographic targeting. The U.S. intelligence community believes Russia obtained much of its information when the Democratic National Committee was hacked last summer; as The Wall Street Journal reported earlier this year, at least some of that voter data made its way to a lesser-known G.O.P. strategist, Aaron Nevins, via the alleged Russian military intelligence front Guccifer 2.0. (Nevins later posted that data, which included an analysis of battleground states like Pennsylvania, on his HelloFLA blog.)

The psy ops campaign described by Facebook, however, mostly predates the D.N.C. hack. Facebook Chief Security Officer Alex Stamos said in a blog post later Wednesday that most of the 3,000 Russian-linked ads it sold between June 2015 and May 2017 didn’t reference the election, Hillary Clinton, or Donald Trump. The ad buy was also relatively small, relative to the impact of fake news posts and misleading information in the media. Only about one quarter of those were geographically targeted, and of those, more ran in 2015 than 2016. But they wouldn’t have needed voter data to be effective. According to the assessment of U.S. intelligence agencies, Russia wasn’t initially focused on boosting Trump specifically but rather sowing discord in the early months of the election. That tracks with Stamos’s conclusion that the Russian ad buys “appeared to focus on amplifying divisive social and political messages across the ideological spectrum.”

Russia didn’t need an inside man to see just how divided the country was in 2015 and 2016, or how ripe the public was for misleading ads and fake-news stories. As The Wall Street Journal reported Wednesday, sharing the results of a new survey conducted with NBC News, the U.S. is more polarized than ever on social issues like immigration, religion, race, and gun control, leaving partisan biases deeply vulnerable to exploitation (emphasis ours):

The wide gulf is visible in an array of issues and attitudes: Democrats are twice as likely to say they never go to church as are Republicans, and they are eight times as likely to favor action on climate change. One-third of Republicans say they support the National Rifle Association, while just 4% of Democrats do. More than three-quarters of Democrats, but less than one-third of Republicans, said they felt comfortable with societal changes that have made the U.S. more diverse.

A separate poll from May found that about half of working-class Americans agreed with the statement, “things have changed so much that I often feel like a stranger in my own country.” Those divisions preceded Trump’s election, and they don’t just show Americans disagreeing on policy or politics—they show Americans inhabiting completely different worlds of values and, increasingly, objective facts. Russia wouldn’t have needed hacked data to push those pain points.

What Facebook’s Russia problem really underscores isn’t how powerful Moscow is, but rather how weak and divided America has become. That Russian operatives could so effectively engage Americans with tactics as simple as Facebook ads, fake-news postings, and Twitter trolls, shows how dangerous misinformation is in the social-media age, when propaganda can be so easily amplified. More important, it highlights how primed Americans already were for manipulation.