(Part 3) What the Fourth Succession War Implies About Population Size

Looking at that same Wikipedia again, it looks like the Allied armies (primarily the Soviet Union and China) suffered about twice as many casualties as the Axis armies (primarily Germany and Japan) did.

Supposing that the 4thSW's 100 million deaths are primarily infantry, and supposing the defenders suffer twice the casualties of the invaders, then defending planets lose about 5 million infantry per month while the invaders lose about 2.5 million per month. If you figure 2.5 million guys at 5 tons each, and a Mammoth (or half a Behemoth) at 40 kilotons* of cargo, that uses only about 320 DropShips (per month of travel), out of thousands of such ships sphere-wide.

As for the defenders, I count significant fighting on roughly 140 worlds, including:

Confederation- Tall Trees, Zurich, Aldebaran, Liao, Bharat, Tigress, Tikonov, New Hessen, Styk, Wei, Truth and Sarna. (The Duchy of Andurien attacked some worlds too, but it's not clear which ones.)
Combine- Buckminster, Kirkbach, Radstadt, Altair, and Vega.
Suns- Marduk and Kathil (whose populations I don't know), Algot, New Aragon and Halloran.
Commonwealth- Tamar and Poulsbo (whose populations I don't know).
Free Worlds- Procyon.

These 25 worlds have a combined population of somewhere near 65 billion people; at the Tikonov ratio, that makes 492.4 militia regiments. Supposing the other 115 worlds have two battalions apiece, that would make 569 militia regiments in all.

From HM:FWL*, an infantry battalion consists of 400-500 people. 7 troopers per squad and 36 squads per battalion would be 252 troops, and a tooth:tail ratio of 1:1 would bring it up to 500 troops. (The 400-person battalion may be Jump infantry, which use smaller platoons.) At 3 battalions per regiment, my estimated 569 militia regiments would amount to 860k troops. With losses of 5 million troopers per month, these militia units would seem to have an expected life span of five days. Or to put it another way, a population of 44 million has a standing force of about 1000 troopers, and can call up another thousand every five days. This is something (I can't find consistent data on it) like 6x-8x slower than nations mobilized for WW2.

Of course, the tooth:tail should be more like 2:3 or even 1:3 instead of 1:1, many of those worlds won't have a full 44 million people, I'm arbitrarily excluding the House's regular army and navy, and I'm ignoring how late the fighting started or how early it ended on any given world. So the worlds attacked in the 4thSW may well be mobilizing at WW2 rates.

*Curiously, that makes the Mammoth's 40 kton cargohold just the right size to carry a FWL infantry division.

(Part 2) What the 4th Succession War Implies About Population Size

tl;dr? Soldiers in the 4th Succession War died at the same rate (per billion people, per month) as soldiers in World War II.

Per Volume II of the NAIS Military Atlas, the Fourth Succession War left 100 million dead and 600 million wounded from thirteen months of fighting. (The war lasted from mid August 3028 to February of 3030, with three months of inaction in the middle and two at the end.) That's 7.7 million dead, plus 46 million wounded, each month.

Per Wikipedia, World War II lasted 68-71 months during which 21 to 25.5 million soldiers were killed (with 8 wounded for every 7 dead), 29 to 30.5 million civilians were killed by strategic bombing or warcrimes, and 19 to 28 million civilians were killed by war-related famine or disease. Earth had about 2.3 billion people in 1940, with about 2.0 billion in countries which participated in WW2.

• WW2 killed or wounded about 630k to 800k soldiers, or .032% to .040% of participating populations, per month. The 4thSW's 100+600 million dead and wounded, at those rates, would suggest a population of 1.7 trillion to 2.2 trillion for the 4thSW. 
• WW2 killed about 300k to 380k soldiers, or .015% to .019% of participating populations, per month. The 4thSW's 100 million deaths, at those rates, suggest a population of 533 billion to 676 billion for the 4thSW.
• WW2 killed about 970k to 1.2 million soldiers plus civilians, or .049% to .062% of participating populations, per month. The 4thSW's 100 million dead, at those rates, would suggest a population of 162 billion to 206 billion for the 4thSW. 

We know that last estimate is too low. Firstly because the Inner Sphere has at least 450 billion people, and secondly because the 4thSW didn't purposefully attack civilians and industry the way WW2 did. (I notice that the old Succession Wars board game has industry get damaged in about 30% of assaults, though.)

The high first estimate would mean an average of a billion people per named star system, which is difficult to reconcile with other data from the period. It also assumes no civilians were injured, that 31st century weaponry is not more powerful than WW2 weaponry, and that improvements to battlefield surgery have far outstripped improvements to battlefield weaponry.

The middle estimate makes the 4thSW at least as deadly for an average soldier as WW2 was, which seems like something the old developers would do. It's weirdly close to the geometric average of the high and low estimates, which is a bit disquieting because I don't see any reason for it turn out that way (especially if WW2 casualties have been re-estimated a few times over the years). Maybe it has to do with how they arrived at 600 million for the 4thSW's wounded.

Then there's the question of how many worlds actually "participated" in the war. I count significant fighting on only 140 worlds, but others contributed soldiers, others suffered famine because shipping was diverted... all five Successor States participated in the war, albeit unevenly; I'm willing to let it wash, given the countries involved in WW2 didn't participate evenly either.