Ukraine

Honestly it don't think it matters much if they hold it in the long run. Ukraine isn't likely to be trying to annex it, and it forces Russia to divert resources from other fronts to handle it. If Ukraine can make retaking it very costly then even if Russia succeeds, Russia has had to spend a whole bunch of lives and materiel to gain literally zero Ukrainian land

Embarrassing Putin, opening up a new front and gaining a valuable piece of territory for potential negotiations for the return of annexed Ukrainian territory.

Not to mention the morale boost to Ukrainian troops.

This is far from the "end" for the Kursk incursion. I expect Ukraine to hold the territory for quite some time as the war within their own borders has shown that fortified trenches and defensive lines eventually lead to static frontlines. These gains were possible because their force is highly mobile in barely defended areas. Russia can't put up any defenses close to the invading force because they would be in artillery range and have to put up their lines way beyond what Ukraine currently controls.

We'll see if the Russians eventually resort to human wave tactics in Kursk, which seems to be the way they approach fortified lines. But even then it will take them a long time to make any progress and they currently don't have the manpower needed for such assaults.

The AFU is very selective in what it is targeting and the initial attack was to limit Russian gas exports and starve them of money. Sudzha is exactly the point that they needed to hold to do that and they did it with surprisingly low number of troops. Russia still has to move troops into the area and the quality that they are sending means there is little expected chance of escalation for now. There is minimal air and ground assets being assigned to that front, as they are scattered across multiple other fronts in Ukraine.

but to what end.

The AFU has created a situation that Russia cannot ignore or back away from. They simply have to send the men and equipment to try and force the AFU out. If they don't the AFU can, and likely will, continue to gobble up Russian territory and that will eventually impact Russian logistics enough that they won't be able to continue to fight in Ukraine.

Russia has already tried responding with dribs and drabs and the AFU simply chews them up; that means in order to successfully counter and push the AFU out Russia is going to have to show up with mass. Many thousands of men and lots of materials all at once, which will make a big juicy target as they try to enter the battlespace.

If Ukraine can get the necessary equipment into the theater, and so far it looks like they can / are, they will absolutely CLOBBER the massed forces that Russia has no choice but send. When I say "clobber" I'm talking about the destruction that Rocket Artillery, Howitzers, and Jet Fighters could have delivered early in the invasion when Russia was sending miles long convoys straight down highways.

Think "Highway of Death" between Kuwait and Iraq from 1990.

Dunno if that will happen but the way Ukraine is setting this up it's within the realm of possibility.

Why would they massacre?

• The Ukranians take as much territory they can before the russian defence materializes.
• Meanwhile they prepare to dig in and require the Russians to take back the territory in the only way they know how to.. grind down defenders with overwhelming firepower and meatwaves destroying everything in their way.

For Ukranians the change is that the Russians will be levelling their own towns here.

If they can capture several 100's of conscripts the Ukranians can trade them for their own people.

And lastly, the whole talking point "accept the realities on the ground" and "negotiate for peace" is dead.. because we'll.. Russia will not negotiate or accept the realities on the ground. Maybe Ukraine can offer to trade the Kursk territory for the restoration of the 1991 borders.