How does Putin's war not turn into a protracted insurgency?

Insurgency is only an issue if you stay for extended periods of time trying to win "hearts and minds" like the US did in Iraq and Afghanistan.

Putin has stated his objectives in Ukraine very clearly:[1]

> Russian President Vladimir Putin has announced that he ordered his country's military to conduct a special operation in the Donbass region after the leaders of the breakaway republics asked Moscow for military assistance in response to what they claim is an increase in “Ukrainian aggression.”

> “Circumstances require us to take decisive and immediate action," the order reads. “The People's Republics of Donbass turned to Russia with a request for help. In this regard, in accordance with Article 51, part 7 of the UN Charter, with the sanction of the Federation Council and in pursuance of the friendship treaties ratified by the Federal Assembly and mutual assistance with the Donetsk and Lugansk People's Republics, I have decided to conduct a special military operation,” Putin stated.

> At the same time, in an address to the public, the Russian president said he wanted to "demilitarize" and "de-Nazify" Ukraine. According to him, "we have no plans to occupy Ukrainian territory."

I would think of it as:

* permanently secure the borders of the DPR and LPR

* reduce the size of or possibly even disband the current Ukrainian military

* deal with Nazis that operate with government sanction[2]

* install a friendly government in Ukraine

It is always possible that someone will do something to escalate the situation that will give Putin the excuse to implement some other strategy.

[1] Putin announces 'special operation' in Donbass (https://www.rt.com/russia/550408-special-operation-putin-don...)

[2] Azov Battalion ( https://en.wikipedia.org/wiki/Azov_Battalion)