Military airports

What happens when you destroy all military airports of a country?Isn't it better,easier and cheaper to destroy the airports?Will that render the airforce useless?

I've never attacked a player before, and c3 countries at low levels almost never attack back. But, I will say that destroying the air force units is worth a few points.
As you know, you have to reduce a country's defense to zero to win the war. If you leave it's military units alone, you will get them when you win. But that comes at another price - you had to destroy something else for those points. Probably, you had to destroy more corporations, or kill more civilians.
In my opinion, destroying all fortifications is costly, but it's worth doing so you don't have to kill half the population, or all the factories, etc..

It depends on what you plan to do once you win. If you want to keep the country, or if you're after the population, then it makes sense to do as little damage to the civilian population as you can. If you're just after resources, then bomb all the cities.

I've always attacked the airports defenses first, then destroyed the airports. After doing that, I've experience almost no losses destroying the air force units themselves. And the first few are worth some points. But knocking out the defenses around airports can be quite costly.

For C3s I take out fortifications, paint, and start on the factories. Since I kill some civilians when hitting the factories I win without having to hit them all or any cities directly. I usually pick from the list and destroy the crap factories that I would end up closing anyway. Once I win I sell all the military units that didn't defend.

Attacking the bases will destroy whatever weapons are there. I would say good idea if it's another player, but like jdlech, I have only hit C3s so far. I don't think you can completely destroy them though. I think I read somewhere in the documentation that the last one you hit will keep 1% health to not be destroyed. You can blow up all their aircraft though, so yes, destroy their offensive aircraft before they can attack you and they will be forced to buy more & create new units while your army runs them over.

There is no use in attacking the airports of c3s and it is seldom useful in attacking a player. Attacking fortifications of c3s is expensive and should only be a target when you have exhausted attacks against corporations and cities, have painted a country, and still need a few more damage points.