In my opinion the West is already doing a good enough job at polarising itself, doesn't need much help from the enemy.
Well empirically it's actually pretty cheap and easy. What's expensive and hard are the countermeasures against it.
To manipulate politics you mainly need to generate fake social proof. To do it effectively you mainly need to target a relatively small number of counties in battleground states.
Have you ever heard of AIPAC? Rupert Murdoch? The UAE?
Chainsaw dismembering princes getting a free pass with their weapons purchases?
If you're only into corporate news, there's Russia in 2016 scapegoating for terrible decisions from the DNC.
But assuming you're asking in good faith the obvious one is Russian interference in the 2016 election, which the former head of the NSA considers
> the most successful covert influence operation in history
Another former director of the NSA said
> it stretches credulity to think the Russians didn't turn the election
The Wikipedia article is a good entry point if you're not familiar with the details [0].
After the election, the Russian government had scandal-making influence in many positions including the National Security Advisor.
The US seems to believe these tactics work because we use similar ones to influence foreign governments. For example you can read about US involvement in the Arab spring [1]. A lot of the techniques are the same, but things weren't quite as online then.
[0] https://en.wikipedia.org/wiki/Russian_interference_in_the_20...
Domestic enemies came to the same conclusion.