Why do you need an air force?

Winning air superiority

Air superiority is either yours or the enemy's. It is only neutral against a minor, and even then rarely. Take the skies and the enemy eats penalties; lose them and you are the one eating penalties.

One important nuance: air superiority gives you no direct bonuses. It only makes things harder for the enemy:

• up to −35% Defense and Breakthrough in combat with strong air superiority;
• −30% movement speed for divisions on the front line.

How heavy those penalties are depends on how strong your superiority is: the more planes you have in the region, the harder life gets for the enemy. Two kinds of anti-air help:

• Anti-air battalions in a division cut the Defense and Breakthrough penalty by up to 75%: in the best case the −35% penalty drops to −8.75%. Every extra battalion adds less and less value.
• Zone anti-air works more bluntly: every building level strips 5 air superiority points off the enemy, but those buildings can be destroyed.

There is also direct AA fire: every point of anti-air attack adds +0.5% to the chance of shooting down an attacking plane, on top of a base of 7%.

The top players build fighters and Close Air Support even as a minor. They know that in this game the air force can decide battles.

Destroying enemy troops

Do not stop at fighters. Build Close Air Support or naval bombers and you can do direct damage to enemy troops. CAS planes shred enemy divisions apart: they visibly melt in front of you when your planes are good, and in HoI4 that is a genuinely satisfying sight. A defense that looks impossible to break, like the Maginot Line, turns into a small obstacle that gives way easily, especially next to tanks.

You get two advantages here: a direct Attack bonus from CAS planes, and an indirect one, as enemy organization and combat strength melt away under those strikes. CAS planes do not care about Defense, enemy equipment or anything else. They deal direct damage, and a lot of it.

Naval bombers hit just as hard. For example I recently destroyed a large part of the British fleet playing as Germany and crossed the English Channel using almost nothing but naval bombers.

Destroying enemy buildings

Tactical and strategic bombers destroy enemy buildings and supply. You can strip the opponent of trains and trucks, or you can wipe air bases and military factories off the map.

With plenty of good strategic bombers plus air superiority, you can turn the enemy into a useless scrap of land: strategic bombers level every factory and the rest of the industry down to the foundations.

Paradrops and Air Supply

To paradrop you need transport planes, otherwise the drop simply cannot happen. The drop requires at least 70% air superiority in the landing region. Transport planes must be covered by fighters, or the paradrop penalties become brutal.

And a deep advance needs transports too: the «Air Supply» mission needs planes as well, and it happens to be a good one. So what happens if you build no air force at all?

Enemy air superiority

Skip aircraft production and all of that trouble lands on you at once. Even a small enemy air superiority gives your divisions −35% Defense and −30% movement speed, and that means hours lost advancing and attacks that fall apart halfway.

Solo against bots it is merely annoying. Against a real opponent, no air force is an almost guaranteed defeat: the skies belong to the enemy, your divisions fight under penalties, and their bombers work without interference.

Your industry and your troops destroyed

Enemy aviation happily bombs your divisions and buildings: factories, air bases and infrastructure burn, and divisions melt under bombers and CAS planes. Even a mighty army suffers losses that make no sense without cover from the sky.

Taking the enemy becomes harder

Without an air force you never unlock the full potential of your country and your divisions: no air superiority means no penalties on the enemy, and no CAS planes means his defense is far tougher, so breaking it costs you many times more lives. On top of that, paradrops do not work without transports. The example below shows how much this matters.

I deliberately played one game as Germany to check casualties and the speed of the advance. The first run had air superiority, CAS planes and Air Supply. The second had no air force at all. Here is the result:

The numbers look made up. They are not. This is exactly how the air force works in HoI4.