Ukraine

making no provision at all for serious border defense may not be most manically immoral, but it may be the most militarily stupid.

That's mostly false, but the truth is significantly more hilarious. Russia made some pretty extensive border fortifications around Ukraine, including the current incursion area, for example: https://read.bradyafrick.com/p/russian-field-fortifications-in-ukraine

Granted, most of the work was done in the occupied areas, because bulldozing the countryside and towns of someone else's country is preferable to your own, but the Ukrainians had to pass some pretty large defensive works.

The engineering is, mostly, not the problem.

When Russia invaded Ukraine, it became super obvious that their conscript soldiers are mostly untrained, poorly disciplined, have terrible morale and generally suck as soldiers. Their contract soldiers are much better trained, and they're mostly volunteers backed with some veterans who imparted at least some skills (Russian units are expected to train the recruits that arrive themselves).

As a result, Russia deployed their skilled soldiers in the areas of heavier fighting, which is a sensible choice. Attacking is a lot harder than sitting in a trench for a few years, after all. In ye olde days of WW1, these conscripts would have been called fortress troops, not trained to attack, only to defend their fixed position.

But Russia also didn't do that. Their border troop conscripts were, as with the original invasion, mostly shit. And if you pour shit into solid concrete fortifications, it's still going to drip out when you shake it, and that's exactly what happened. Again. For the third time this war.

So Russia can either pull their contract "volunteers" from the front to sit on the border, meaning they can't attack as well, or they can start losing troops and land to these kinds of attacks. Either way, sucks to be them.