If you're not so pacient on taking diplomacy with everyone and likes war as this game is dedicated, welcome, this post may help you with some war tactics on this game

1 - Let them attack: first of all, if you build a good defense against a weaker and smaller nation (Example: China vs North Korea) Let them attack until their economies are struggled, (this may be noticed if they attack with less troops), then after all the attacks, strike violently against the enemy and win the war with low resistance.

2 - Don't be sieged: NEVER let any enemy siege your army, it may makes it without supplies and weaken army, you may lose alot of troops, that is not glorious and the enemy may push back against you, also do sieges against strong armies of like 2000+ soldiers to weaken the enemy.
3 - Allies: Allies are the cherry of the cake in this game, with allies together both of you can beat a stronger nation (example Camboja and Laos vs Thailand) they also have a nice force that may help you alot in battle, also if your army is completely destroyed, your ally can still hold the positions when you're recovering your army, i've used this tactic alot of times and worked pretty well.
4 - You're never overprepared:  Thinking that you're overprepared for any war can be some kind of illusion, (don't think like you can't take luxembourg with 70000 troops, please)  this game is a box of surprises that can destroy you some time (example: 3 nations may declare war at you at the same turn), you need to have absolute trust that you're going to take that country.
5 - Have a good economy:  To have a good army, you need first to have a very well developed economy,  so you can still be stabilized with an enormous army spending, this may help you to recover the army in case of emergencies, if not have a good economy, VERY HIGH CHANCE of your treasure be negative and you lose cause you can't rebuild the army.
6 - Armoury and Supply camps: NEVER forget about these 2 buildings, they ABSOLUTELY are important for your wars:
 
Armoury: Basically troops become cheaper, very good for builds of enormous armies
Supply Camps: Very important if you're invading a big country (Example: invading russia)


Thank you all for the attention, Hope i helped you alot, if you got any extra tips, comment it, it may help new players to get good at this kinda complicated game.

- Yetaloz

I was thinking about an idea where based off of how many troops you have compared to the enemy it would take off a percentage + a bit extra. Instead of it being subtraction based where battles occur instantaneously; this will allow you to hold off/take longer to invade while you/the enemy has some time to get reinforcements.
      And a bit unrelated but Forts also should be modified to be more powerful.
 

There is a serious problem when it comes to technology in the game, and it is ticking like a time bomb.
Technology in the game is grossly oversimplified, and in a lot of strategies largely ignored.  This post will explore ways to make the game more in depth, realistic, and balanced.  It will be split into two subs:  Balance and military.
BALANCE
When it comes to technology, the game is broken.  Every time, the bigger nations with the bigger economies are leaps and bounds of the smaller nations, say the HRE.  This is a joke, as regions should develop in technology uniformly.  Development of technology should be not be dependent on income, but instead possibly a fixed speed. It could be increased with events, which I already do in my Too True 1440 campaign.
Of course the problem with this is the player loses the freedom necessary to say, build an Indian colonial empire.  As I said earlier, I have events in my campaign giving tech points to certain civilizations.  Say, the Renaissance or printing press.
There is still much to do in this topic.  Any suggestions to replace the income system?
MILITARY
This is an in depth look on specific new modifiers that could added to invest technology points in.  As of now, all seven modifiers basically increase your income.  This means people do not care as much about it, and rightfully so.  There should be added new modifiers affecting the outcome of a battle.  These include:
Offensive modifier
Defensive modifier
Fort modifier
Siege modifier
Fort maintenance modifier
Navel battle modifier
These new 6 modifiers would greatly change the game in a good way.  Now this would add realism, where a smaller European army could destroy a larger Chinese army.
This list can still be worked on, so feel free to comment any changes!

Instead of building a army using money, scrap that and have it being that it require that province's population to build a army.
This will make so that your gold can not quickly build a army in one turn. 
Example, you select provinces that will build a army for you based off of the population the province has.
Population of 25000, would be able to recruit a army of 1500 but it would take 3 turns.
Each turn makes 500 units
There needs to be a new building so that you can train a army. The higher the level the building is the faster and more units you can build.
The cost would increase the military upkeep.
This would allow a nation prepared for war, have a higher chance of winning.
Even if you are a small nation but have time to build up a army without costing gold would be ideal in terms for war.
A Big Nation will always out produce units because of gold but take that away then this allows a small nation to take down a big nation or at least get them to agree to a peace deal in the small nation favor.
Having these buildings in the game would allow strategic planning since now you know where you should plan your attack.
You would want to take out their building so they cant counter attack or build a army close enough.
With what we currently have a Nation with a high budget can build a army of 2500 in one turn, which is annoying because you probably was prepared for war but suddenly they out number you.

I have an idea. One could add such a parameter as the strength of the troops. It depends on the level of technological development of the country. One could also add the possibility of creating some kind of new building that will increase the strength of the army. Because of this parameter, it will be possible to defeat 1000 troops, who have a small force, with the help of 500 units of troops, who will have great strength.