Land invasion (can be used in conjunction with airstrips for targets up to 3000 km away)
Basic setup:
250 Anti Tank Missile Batteries - 10k ammo
1 000 Mid Range Missile Batteries - 10k ammo
1 000 Heavy Artillery - 300k ammo
1 000 Heavy Tanks - 300k ammo
rest, Fighter Planes, Attack Drones, Special Forces, alternatively Navy Fighter Planes as in previous set up***.
As usual "paint" with SF. In the mean time move LRD consisting of the mentioned in, close to a County or a Town, when in place use "attack defenses" with FP/Drone combo to draw out defending Interceptors and Helicopters. Only Drones will take losses, in two attacks, of around 400 Drones and only FP will use ammo.
When defensive air force is taken care of, LRD is free to do its job.
Personally I go for Capital first, attacking defenses with Anti Tank Missile Batteries and Mid Range Missile Batteries. This is where LRD will take heaviest losses but still quite reasonable of around 35 MRMB. Once defenses are destroyed Heavy Artillery and Heavy Tanks are can destroy the target in 3 hits for cheap.
Then I move to destroy Counties and Towns simply because they don't have garrison and can be yet again destroyed with HA/HT. In the mean time SF move around the country to "paint". After Counties and Towns are destroyed, target country is mostly "painted", WI should be pretty low.
Now we have basically 2 options. Attacking Cities or Factories. Since both have same garrison the fastest and cheapest way to win is to go for Cities, however, it might be desirable to go for Factories, their cash and workers, instead. Either way, attacking defenses first with Batteries, MRMB will take around 3-8 units of casualties each hit, then with HA/HT combo to destroy target. This way its possible to get the WI down to 0 without ever touching Cities or Factories, and their defenses, depending on our preferences.
Some math at the end, actual casualties and ammo usage when taking c3 today. Mind that "your" numbers may vary depending on several factors like targeting priorities, but it should not vary too much.
Casualties:
407 Attack Drones
61 Mid Range Missile Batteries
Ammo use:
4 647 Air To Air Missiles
847 Anti Tank Missiles
2 949 Mid Range Missiles
141 271 Heavy Artillery Shells
121 995 Heavy Tank Ammunition
Cost of unit setup (market price at time of posting):
250 Anti Tank Missile Batteries - 7 660 000
1 000 Mid Range Missile Batteries - 35 380 000
1 000 Heavy Artillery - 17 630 000
1 000 Heavy Tanks - 20 720 000
total: 75 645 000 000
Cost of ammo setup (market price at time of posting):
10 000 Anti Tank Missiles - 33 300 000 000
10 000 Mid Range Missiles - 196 800 000 000
300 000 Heavy Artillery Shells - 35 686 200 000
300 000 Heavy Tank Ammunition - 36 336 000 000
total: 302 122 200 000
Total cost of unit setup + total cost of ammo setup = 377 767 200 000
Cost of casualties (market price at time of posting):
407 Attack Drones - 10 284 890 000
61 Mid Range Missile Batteries - 2 158 180 000
total: 12 443 070 000
Cost of ammo (market price at time of posting):
4 647 Air To Air Missiles - 79 091 940 000
847 Anti Tank Missiles - 2 820 510 000
2 949 Mid Range Missiles - 58 036 320 000
141 271 Heavy Artillery Shells - 16 804 750 534
121 995 Heavy Tank Ammunition - 14 776 034 400
total: 171 529 554 934
Total cost of casualties + total cost of ammo: 183 972 624 934
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***the mentioned setup was for using Precision Bombers instead of Batteries to attack target defenses so this setup lacks 1000 Fighter Planes, 1000 Drones, Special Forces, plus ammo, in description and in setup costs, but not in casualties and ammo usage costs. In similar fashion Transport Planes are not counted in. Sorry about that but I'm too lazy to add those.