My basic strategy. (very basic)

The following are some basic strategies I use in any match. They are not the only way, but I've found them to suit my style of play.

First two days (are key):
1. Start researching 2 technologies. I usually do infantry and AC. Ensure you have two techs under research nearly all the time.
2. Build the following in the 5 starting IC provinces in this order, barracks, infrastructure, factory.
3. Simultaneously start production on as many infantry as possible, hopefully you can afford 5.
4. If you can afford it, build infrastructure in resource provinces. I usually focus on resources I'm low in at the start.
You won't always be able to do all the above but if you can get the first 3 done you should be in good shape. Plan ahead as well, you may want to hold off on some infantry and infrastructure improvements to keep the tech research going, those initial quick researches can eat up goods.

Consolidate forces and look for easy kill minor nation but don't leave your core provinces vulnerable. Whatever you do, do not overextend yourself, especially by taking unnecessary non resource provinces. Every province drains food and goods, so make sure you don't outstrip your supply by needlessly increasing demand.

Tech research: I can't overemphasize the importance of maintaining technology research. It's the one aspect of the game that once you are behind it becomes impossible catch up. That being said make sure you are researching technologies you will use. I try to get light tanks as soon as possible, in anything but cities they are very good to decent in the early game. I also research artillery and interceptors while continuing to improve infantry as it becomes available. On day 8 I highly recommend researching Commandos, they absolutely kill in the mountains and are really tough in urban terrain. Be flexible, and research the technologies you can easily produce first. Much can change depending on the starting country.

Early attacks: I usually don't attack before day 2 or 3 just because it takes so long to consolidate initially. That usually means I have 4 or 5 more infantry than at start. Early attacks are slow but relatively easy. Attack at 3 to 1 or better odds. Remember the weakness and strengths of your units. Try to take out a 3 or 4 territory country first and take the capital last if you can. The 10 bump in province moral will allow you to leave new territories un-garrisoned if you can take them all before new day. ( I always plan my first attacks to take place shortly after new day so that I can finish before next new day. I hate paying the morale penalty for wars.) If you are lucky enough to have more than one minor country on your border, take care that you don't overextend, your neighbors maybe waiting for you to finish off the minor forces and then declare war on you.

It is incredibly important to balance your units. In CoW there is huge strength in diversity, not just because you have the right tool for the job, but state-based damage efficiency is a huge penalty to a stack made up of too many of one unit

Very true.  Here are some stack limits before you start paying damage efficiency penalties.
Infantry, Motor Infantry, Commandos, Artillery: 8
LT, Mechanized Infantry: 6
MT, All air (except strategic bombers which I haven't built one of): 5

I suspect anti-tank, anti-air to have the same stacking limits as infantry and that self-propelled variants would have stacking limits similar to the tanks.

As far as naval stacking the only units I've tested were subs and that limit is 8.  I have tested destroyers now, limit is 7 before incurring overstack penalty.

hello im testing