For the air part, I think the best is to start by reading this guide:

forum.callofwar.com/index.php?…ng-some-in-decent-damage/

which explains why your tacs will become significantly more efficient when fighting in stacks.

The link that artreas has sent doesn't work, go to following link: forum.callofwar.com/index.php?…ng-some-in-decent-damage/

It's classic that a lot of player go for tanks and it's easy to counter. Produce tactical bombers, they are the fastest and most powerfull units in the game. I usually have minimum 25 interceptors and 25 tactical bombers. The interceptors are important because they protect the bombers and they absorb more damage so the tacs last longer. Make them in stacks of 5/5 for optimal SBDE and patrol over the enemy tanks. Tank players usually don't have AA or not a lot so you kill them easily. With planes in attack and a couple of AC, LT you will expand really fast.

Combine this with MT and maybe tank destroyers to lock those units in combat so that they can't escape. While locked in combat hit them with a stack of 8 SP arty also max SBDE. You will kill almost any enemy with very little losses. Your planes can mobilize fast so they are perfect to use against an invasion. Most players never have enough AA to protect stacks.

Go for navy: use submarines to protect your coast and put them on every dot creating a blockade. Produce also some destroyers. With those cheap ships you can take down a strong navy without spending too much resources or time on knowledge. Lock ships in combat with subs and when locked in combat bombard also with destroyers they will bombard without being counterattacked. You will lose submarines but it doesn't matter because they are really cheap in both manpower and resources while those ships cost a lot. A lot of players waste too much on a navy and underestimate the combination of subs and destroyers.

Well, one can really write a whole article about navy battles (there are even a few good online videos on that subject), but generally sub strategy relies on approaching the enemy capital ships. This is not guaranteed (in fact it will 100% fail against any good navy player). Certainly don't even try it without High Command (for some peculiar reason, Bytro has given fire control to subs, although they are not range units).

In fact it is much more "paper-rock-scissors" than that. For example, in the above scenario that BMFox described any good naval player would have put his subs ahead of the capital ships, engage enemy subs, kill them with his DD, and if the enemy DD dare to approach kill them with CC/BB. It simply doesn't work, unless you manage to engage the capital ships with your subs.

I've just described a scenario that works if you don't want too invest much in navy. This tactic is better then no naval at all. Yes this tactic is better against offline players. At the other hand i haven't met a lot of players yet that are good naval commanders.