This scenario assumes that Humans somehow invaded Kerbin, and having the Kerbals militarize due to the Human invasion, Humans would have modern technology, Kerbals would have the entire stock tech tree technology. (Why? I don't know)
This is also using the theory mentioned on the KSP Discord, where Kerbals, instead of bones, they have a large amount of liquid bladders inside them, which explains how resistant Kerbals are to falls in-game, I can't remember who made it, I will edit this if I find out.
As this scenario is set at a time when the stock tech tree is completed, Kerbals have the advantage of various bases in space.
This scenario does not have instant construction, as that would mean Kerbals could just fire a rocket a minute at the humans.
This scenario would have KerbinSide.
This would be Career mode, so there would be funding, in this case it's reasonable to guess that Kerbals would have massive amounts of funding, as Kerbalkind would probably not like being invaded by Humanity.
For Kerbal militarization, they could design basic (but powerful) weaponry with stock KSP technology, such as, firing entire SRBs as rockets, and dropping fuel tanks out of planes as bombers, from the start they would have highly powerful anti-tank weaponry, in the form of modified SRBs designed to fit in a shoulder-mounted tube.
Humans have the advantage of being militarized throughout their entire history, therefore having advanced weapons, nuclear, chemical, etc.
Now, onto the scenario.
At first Humanity would have the advantage, having more advanced military technology, they would rapidly occupy land, after a while, Kerbals would start building rockets just to launch at the Human military bases, and then they would start launching planes to drop fuel tanks onto the Human forces.
After a while, presumably from reverse-engineering Human technology, Kerbals would develop Human-like guns and cannons, Kerbals, having the ability to build massive stuff, would likely make massive artillery cannons, using those instead of rockets to fire at the Humans would be quicker, due to only needing to make chunks of metal filled with explodey stuff.
Both sides would likely do bombing raids against the opponent, the Humans would have interception and AA technology from the start, Kerbals would develop those eventually.
Kerbals could modify NERVs to work as basic Nuclear bombs, although, due to the smaller size of Kerbin, nukes could cause serious(er than on Earth) issues, so they probably wouldn't be used much, if at all.
Kerbals would use space stations to bombard the Human-occupied surface with Kinetic WMDs.
There would likely be a stalemate between the 2 sides, with the Humans losing large amounts of resources and troops, with Kerbals not being able to produce enough weapons to repel the Humans completely.
At the end, either there would likely be a ceasefire, and agreements to end the war, or eventually the Humans would run out of soldiers, resulting in a Kerbal victory.