Thread by @ArmchairW

Let's talk about defense in depth. Seems appropriate at the moment.

A thread:

Most field manuals will give you a map like this and talk in circles around how this tactic is actually supposed to work, probably because most soldiers (with good reason) don't particularly like admitting to trading lives for space and time.

Regardless, it is simple, easy to execute, and works extremely well.

Everyone is all gung-ho to do a slick mobile defense until they find out that if they miss the counterattack timing by an hour the entire plan falls apart.

The standard way to deploy for this (and for many other missions) is with a "two up, one back" arrangement, where a unit deploys two of its three subcomponents forward with one in reserve.

This provides adequate coverage of the front line with an adequate reserve.

Here we see a "heavy" division deployed in exactly such an arrangement, with two brigades forward and one in reserve, and each of the forward brigades deploying two of their battalions forward with one in local reserve.

A screen is deployed forward of the defense proper.

As is noted on the template, each battalion on the front line has ten kilometers of frontage - quite easy to cover for a heavy unit. This works out to a line of front-line platoon battle positions a kilometer or two apart, simple enough to cover with patrols and heavy weapons.

This unit has also had plenty of time to dig in and has established three separate obstacle belts.

Each of them is a complex obstacle that will require hours of work by sappers to breach.

But we're not fighting this as a division, we're fighting at the corps level. This is a heavy corps deployed in a defense in depth across a 160km front, with four divisions forward and one in reserve.

This force has a total of 45 maneuver battalions available, 9 per division.

It's an old military rule of thumb that if you want to attack successfully you need a 3:1 advantage over the enemy. Every defending soldier can be expected to incapacitate two attackers before falling himself, and the third attacker will finally take the position.

With that being said, it's astonishingly rare that one army is able to just gain a theater-wide 3:1 advantage. Generally what happens is one army amasses a more local superiority to allow a breakthrough and collapse the front.

Here, the enemy has gained a 2:1 overall numerical advantage, but by massing most of their force against a single sector they'll achieve a 7:1 advantage against the targeted division. The remaining forces will conduct local attacks to tie down the other front-line units.

The screen falls back as the enemy attacks. The first wave in the breakthrough sector is 18 battalions - two whole divisions set against the four front-line battalions at 4.5:1 odds, dropping to 3:1 as the brigade reserves move up.

After an intense battle the first line is breached and the two front-line brigades are wiped out, taking 12 enemy battalions with them. There are modest losses on the flanks.

As the enemy main effort becomes apparent, reserves converge from the flanking divisions and corps.

Now facing 14 battalions in the breakthrough zone, the enemy commits a full 30 battalions to smash through the second defensive line.

More distant reserves are still converging on the growing salient, and the fixing attacks on the flanks continue.

Even this massive force struggles to make headway, but eventually smashes through the second defensive line at the cost of almost every unit on both sides of the main assault rendered combat-ineffective.

All reserves are committed to hold the final line.

Now running out of reserves, the enemy commits their last 18 battalions in an attempt to crack the third line. This time, growing operational friction (they're now attempting a third breach 20km deep into the position) and declining numerical advantage defeat the attack.

The enemy consolidates and transitions back to the defense, with three-quarters of their total force (some 68 battalions) out of action after punching a 40x20km dent into the line.

Losses on the blue side, while lighter overall, are proportionally severe at 32 battalions.

As an aside, Red owns most of the battlefield and will be able to salvage damaged and abandoned equipment left on the field.

Now, it should be noted this is an "all else equal" analysis. Fire and air superiority will both affect the battlefield in critical ways - fire superiority by easing (or steepening!) the brutal 2:1 loss ratio and air superiority by interfering with reserve movement.

How is this relevant to Ukraine? Well, that corps defensive layout is a "doctrinal template" that can be applied to existing terrain as a first step to analyze force dispositions.

The front line in Zaporozhe happens to be about 160km long. Let's see what the Russian defensive scheme looks like with it applied.

It even fits closely into the known Russian obstacle belts. I assume (reasonably) there's a less visible mine belt along the contact line.

Not only can the Russians be expected to rapidly gain and keep fire and air superiority across the battlefield (and move in reinforcements from out of sector), but I don't know where the Ukrainians expect to get the 100+ battalions they'd need to actually defeat this scheme from.

And if you think I'm being optimistic on the Russians here, take your concerns up with EUCOM. Gen. Cavoli's recent lukewarm public comments on Ukrainian prospects going forward speak for themselves.