when "us" is a narrow group. Increases group loyalty, that external threat, and enhances group boundaries.
It's harder to think "they're against us" when "us" includes our foes. Then suddenly have to choose which battle to fight--do we team up with those we despise to counter an external threat or continue to exacerbate divisions at the risk of doing the external threat's job for them?
I think it's worth noting the reports that say the shift from purely "we like Trump" to a large portion of "we don't like Trump" on the part of "Russial media" (Russian-influenced social media) occurred in less than a day or two after Trump's election. What did Trump do to piss off the Russians? Nothing. But if chaos and discord are the goal, as the combined US intelligence report said, then the data track.
Ukraine found this out. The tussle between pro-Russia/pro-Western political forces continued after Crimea annexation and in the run-up to the Donbas fiasco. The result was at that critical point when Russian-backed actors were busy taking over towns, people were debating pro-Russia/pro-Western. Pro-Ukraine was lost in the shuffle until hundreds of people were dying. Helping the fiasco along was a continual bombardment of Ukraine by Russian media and infiltration of Ukraine news and social media sites by Russian disinformation. DU fell for it--there were frantic reports posted here of Russian disinformation like a huge convoy of fascist Ukrainian forces about to invade Crimea.
The problem began years before the actual Donbas takeover. There were Donets'k People's Republic flags flying at paramilitary youth camps for 5, 6, 7 years before the actual takeover. Training teens to use weapons. Forming them into squads. Teaching them that Ukrainians that opposed Russia and Russian were fascists. Nobody shut them down. To shut them down wouldn't be pro-Ukrainian, but pro-Western and anti-Russian. It's not mere framing--it really is a question of what's more important and what's to be ignored.