One detail of the Belgorod operation is that it made the Russians evacuate tactical nuclear weapons from a storage facility in Belgorod Oblast:
www.newsweek.com/…
I suspect that this may have been one of the main supporting goals of the operation, and it would explain the location and limited depth of the incursion.
What was the operations primary goal? I think the primary goal was to force Russian forces to try and defend a much longer front — including the borders of Belgorod and Kursk oblasts. (If they only send forces to Belgorod, then Ukrainian/rebel forces could surround them from the north through Kursk.)
In order to force this, a serious invasion of Belgorod/Kursk must be a credible threat. And naturally, any such invasion must be approved of by Ukraine’s Western allies.
What are the chances that the Western allies would be okay with Ukrainian backed Russian rebels overrunning a nuclear weapons depot? Right. Not high.
In fact, it could plausibly be a secretly negotiated pre-condition that this possibility be taken off the table. How to do that? Well, the best way would be if Russia moved those nuclear weapons elsewhere.
Thus, the Belgorod operation. The duration and scale of this incursion is bigger than previous raids … but small enough to ensure the Russians would “Bagdad Bob” it (pretending it doesn’t exist rather than going straight for nuclear escalation). However, the location and direction of the attack makes it look like maybe the raid intends to capture nuclear weapons. So that finally puts the fire under some Russian butts to evacuate the nukes.
So what has the Belgorod operation accomplished?
Well, it has proven that the Russian border is NOT a nuclear red line, and it has removed an honest to goodness plausible nuclear red line (the nuclear weapons depot). This sets conditions to make future, bigger incursions more plausible, more likely, and more supported by Western allies.
It further minimizes the chances that the next invasion of Russia across the Ukrainian border is treated as a threat of nuclear war. The next one will likely be treated as a joke, at least by Russian propaganda, and it’s hard to do that at the same time as threatening that the only possible response is nuclear war.
It also makes it look, to paranoid Russian leadership, that NATO has told Ukraine to do this operation as a pre-condition of supporting a major offensive into Russia.
Even if Western allies have no intention of ever agreeing to a serious invasion of Russia, the Belgorod operation makes the threat plausible enough that Russian forces are compelled to try and do something about it — and stretch their defenses even further all across the front.