Only three decades ago, the Soviet Union came out with this:-
KGB General Aleksandr Sakharovsky said: "In today’s world, when nuclear arms have made military force obsolete, terrorism should become our main weapon."
That was his analysis as of whenever he said that; which during the cold war would have been pre internet. What would he have been suggesting today?
Simply put, an open and democratic society is obviously much better to live in than a dictatorship and almost nobody denies it. As seen in Ukraine, people are in fact quite keen not to live under a dictatorship and are willing to die rather than submit to this. There are two approaches to dealing with such a problem if your a very closed authoritarian dictatorship.
First is to compete openly at seeing who can do better at certain things, such as quality of life in a manner such as a race (ie like Formula one). The problems are fairly obvious; centralised autocratic dictatorships don't do building things (or accepting feedback) well so they tend to "win" production races over democracies by doing things like eliminating quality control because checks to make sure that the output works hurts the recordable output.
The second option is more akin to a demolition derby, to win by smashing all of the competitors off the track or crippling them so badly that they have to slow down. Autocratic dictatorships tend to do better at these sort of things.
We've seen democracies destruct before in the Weimar Republic at the least, and somebody probably sees that as an ideal aim to head towards because as the quote above, it's not possible to beat us via marching troops through our capitals because we'd get twitchy with the nuclear trigger long before that point.
If you accept the hypothetical than certain nations do not wish us well, how would you go around causing trouble today?
I (personally) wouldn't go funding terrorists these days.
One could add a small number of agent provocateurs to peaceful protests to throw bricks and petrol bombs over the heads of peaceful protesters, in the hope that they wouldn't be so peaceful when the police officers on the receiving end of brickbats decides to break up what they not unfairly consider to be a less than entirely peaceful protest. That'll reduce public support for those sort of protests sharply, while further increasing tensions at the contact points. I'd aim to multiply contact points like this to the point of infinity until things come apart.
I'd fund political extremists on both the far left and far right, on the basis that they both need each other to survive. I'd try and stir up trouble by lavishly funding any group of extremists. I wouldn't do it via handing them a few million from KGB inc; even extremists might have second thoughts under those circumstances about if they are doing the right thing. Nope, i'd employ astrotuf (fake grass roots) by making the handful of extremists think that they aren't a single lunatic in their bedroom, they are part of a worldwide movement and look at the thousands of people willing to donate money via crowdfunding to do X. Easily done via mules in money laundering. Push the extremists steadily further towards the extremes at both ends (left and right), while having both target each other and the middle. Sooner or later it'll cause problems for the majority, who'd be willing to make things less open and democratic to deal with the problems and beyond a certain point both sets of extremists will end up creating more recruitment for each other.
We're probably at about this point at the moment.
The "problem" is that while they are useful idiots for foreign interests, that's not actually criminal, and nor is spreading foreign propaganda from their sponsors. Pointing out that they are being useful idiots of foreign powers probably won't help; not least because there will be a game plan somewhere for using the realisation that people are being played like this to further reduce trust in our societies. (you can just see "Your a foreign funded agent!"; "no YOU are".)
The "best" way of dealing with this presumably would be to very openly pull out financial records of what sort of money laundering is being done by our "friendly" adversaries and go after the money and hope that the recipient's of that funding would go off in different directions if they realise that they are simply considered to be useful idiots by their sponsors, but how would you go about doing that?