This is nonsense (in an otherwise fine article, this paragraph should have just been edited out):
It was a Russian, in fact, who wrote the playbook on the kind of covert warfare which the Kremlin is currently waging in Odessa, Slovyansk, Kramotorsk and Donetsk. Evgeny Messner fought for the Tsarist army and later for pro-Nazi Russians: his 1960 book Insurgency, or the Name of the Third World War predicted that the future wars would be waged by small terrorist cells and special forces, gaining influence by subversion and organised revolutions rather than through traditional warfare.
What Messner was referring to was the Cold War-era inability to pursue decisive conventional military operations under the threat of escalation to nuclear war if you start to lose a conventional war that threatens vital interests. This is why the Cold War was fought in the Third World mostly between Soviet-backed insurgents and terrorists fighting Western-backed governments rather than between conventional forces on the inter-German border.
In the Cold War, even Third World conventional wars between American and Soviet clients had to end quickly lest the major powers get drawn in and then escalate to nukes. In that environment, wars waged by terrorists and special forces fought below the nuclear trigger threshold were the only way to safely challenge the other side.
I've written of this before, especially in noting the growing confrontation between India and China which could be recreating that Cold War nuclear logic. I speculated that India's Cold Start doctrine was really about fighting China under threat of nuclear escalation rather than conquering Pakistan.
Indeed, I've wondered if our quick stop in the Persian Gulf War of 1991 was as much the habit--despite the lack of a Soviet Union willing to go to the brink over Saddam's Iraq--of wanting to end a war quickly while we had the edge rather than risk huge losses by trying for more. By 2001 and 2003, we shed that habit to push for total defeats of the Taliban regime of Afghanistan and the Saddam regime of Iraq, respectively.
Although in this Cold War logic, only the impact of nuclear weapons was new, since supporting dissenting factions in enemy countries is as old as warfare. Just read Thucydides and you'll get your fill of outside-supported rebels. Remember, too, the reason that Athenian man ran all the way back to Athens 26.2 miles from the Battle of Marathon was to make sure that it was known to dissidents ready to open the city gates to Persian troops that the Persians had lost and so weren't coming to the city.
Regardless, the logic of 1960 does not fit to Ukraine of 2014. Ukraine is not in NATO and Ukraine has no nukes thanks to the Budapest memorandum of 1994 (which provided Ukraine with the valued commodity of Russia's written respect for Ukraine's territorial integrity in exchange for Ukraine giving up all their nukes inherited from the Soviet Union). There is simply no credible threat to nuclear escalation over a Russian conventional invasion of Ukraine by Russia, any more than there was a threat of nuclear escalation over Russia's invasion of Georgia in 2008.
Not that Ukrainian nukes would have necessarily deterred Russia. If Ukraine had nukes, then Messner's playbook would in fact apply to a Russian-Ukrainian clash over Crimea and eastern Ukraine. Would Ukraine have risked nuclear war that destroyed all of Ukraine over the loss of Crimea or even Crimea plus the eastern salient of Ukraine? I doubt it. Although perhaps Russia wouldn't have risked it. Nukes are funny that way--the perils of misjudging even a small risk are enormous.
Russia is mucking about in Ukraine with Messner's playbook because there are enough pro-Russians in eastern and southern Ukraine to turn out and arm, and because Russia's Spetsnaz and intelligence people are way better than Russia's armed forces which I judge aren't ready for prime time in a major conventional operation followed by a counter-insurgency campaign.
Other than Twitter, there is nothing 21st century about this campaign.