General David Petraeus is best known as the commanding officer of what came to be termed The Surge in Iraq, a decisive and, for a time, successful effort starting in 2007 to counter the insurgency from Islamist militants that followed the initially succesful invasion. He did a Ph.D. thesis on the Vietnam War, and wrote the US Army's counter insurgency warfare manual.
Given the strong morale of the Ukrainian forces so far (2 days in from the Russian invasion) and the commitment of political leaders, including President Zelensky, to stay the course rather than flee the country, together with the geographical size and population of Ukraine (44 million), Petraeus expects an insurgency to occur and become a real headache for the Russians.
Based on his experience, he thinks the reported number of Russain troops (190,000) will not be sufficient to defeat such an insurgency, given the "soldier heavy" nature of counter-insurgency warfare.
Rao: You’ve studied and practiced counterinsurgency for much of your professional life. Would you expect an insurgency of some kind to emerge in response to any major Russian attempt to control all of Ukraine?
Petraeus: Yes, I would, though it is hard to determine just how large and committed it would be. There are numerous factors that will be important in such a case, the most important of which is whether the Ukrainian government and forces can retain a large part of their country in the event of a large invasion. The Ukrainian parliamentary-delegation member with whom I met on Saturday at the Munich Security Conference assured us that the Ukrainians will fight on, even if, as expected, a full-on Russian invasion might overwhelm the Ukrainian regular forces. The members of Parliament reminded us that, in addition to the several dozen combat brigades, there are also special-forces units and several dozen partisan brigades (made up of civilians, with modest training on weekends) in Ukraine. And they were certain that those elements, in aggregate, would make life very difficult for Russian occupation forces. In fact, they said that Ukrainians would endeavor to make their country a “porcupine” that would be extremely difficult for Russia to digest.