Try to fight each turn to get as much experience for your commander as you can
Higher commander's rank - more units you can hire into an army. Same applicable for enemy commanders, beware them.

Don't wage wars against everyone
You have only one army which can make only one turn. Fighting against everyone means you'll lose more than gain.

Upgrade defense
When num of units in the garrison is equal to num of invaders force it is like 50% chance to repel enemy invasion.

Upgrade defense, pt2
If you're busy in conflict with a neighbor on one side of your state it's very likely you'll get backstab from another one. That's why see hint above - upgrade defense! AI less likely attacks provinces with very strong garrison.

Capitals are undefeatable
Capitals are like rocks or riffs in the sea. All armies (yours and those controlled by AI) avoid invading them. As they garrisons have a max level with 24 units, only at General rank of your Commander it is possible to challenge province owner. Or even at Colonel rank if you show enough level of mastery in leading forces.

If capital is unreachable, how to defeat the country then?
Chase, beat their commander and take their other less protected provinces. When country after a series of defeats and invasions reaches stalemate being unable to recruit enough forces to storm your strengthened garrisons, after some turns passed, it will surrender to your mercy giving to you all remaining lands. But also, at later period of game, one huge decisive general battle is enough.

Invade Russia
Seriously, there is no winter in this game. And their pikemen which can't fire but form one third or almost half of garrison and Commander's army are easy targets for rifle cavalry (chasseurs, dragoons) with hit&run tactics. This makes easy wins at all battles. Invading Russia you can establish a strong industrial base with no neighbors to the south, east and north. You can even allow your enemies to conquer your homeland to reconquer it back, it's affordable if you're playing as Prussia or Poland.

Campaign victory condition...
... is capitulation of all other nations on the map.

At the start of battle capture closest to you coal mines
During battles, your forces spend too much coal. At the beginning of the campaign due to low production power, its number can reduce to zero. It's a really critical point because your soldiers will not have enough powder to shot forcing you to the bayonet attacks of last hope.

Capture other villages too
They gather resources which later in the battle aftermath are added to the country storage. When occupied, various events can be triggered: quests (targets are marked on the map with red flags; reward you with Commanders experience points, or extra soldiers on the current battlefield), fixed amounts of resources (like 3k coal). Also, you'll have no guerilla warfare on your internal communication lines as a local militia loves to shot nearby soldiers moving by the road.

Start your morning with nice workout
When battle just started in first 10-15 seconds policeman guards of villages are leaving their houses and gathering into the unit. In this time they're especially weak and kicking only a few of them is enough to capture village without any losses. Having cavalry allows you to do the same trick even on distant villages really fast.

Use villages to replenish infantry
If some unit suffered losses just move it to the village center and you'll see how new reinforcements are arriving from nearby houses. Also, this makes villages very useful defense points as losses are restored even during the battle. NOTE: not applicable for cavalry.

AI is passive when he protec his region and AI aggressively attac when you defend region with your commander army
What does it mean for you: when you attack - AI just protects strategic points and doesn't care too much about other villages or quests targets, so you can divide your army and do your business in multiple places for your benefit. When you defend - AI will chase you, so try to stick all your forces together at the nice defensive position.

Use hills and forests and rivers
This game is all about tactics so use the terrain for your advantage. Place units on the edge of the forest, so when they're being shot they'll suffer the least damage. Place them on the hills so they'll have a bigger shot distance. Defend rivers crossing and bridges, especially by cannons charged with shrapnel, as for the enemy moving through the water these routes are narrow, but on the land you can have wider front and more units engaging enemy.

Use roads to move your armies
First of all, it's faster. Do blitzkrieg. And when your army moves through the wilderness it completely loses morale.

Train troops on routing enemies
When there are no more stable enemy units nearby let your recently hired units to train on chasing and killin broken enemy soldiers. By this, they can faster promote to the elite "Guard" rank (it's required 300 enemies down) becoming a really superb force on the battlefield. On the opposite end of the map there is also a camp for routed soldiers gathered in a deserted crowd. For especially quick promotions assault that too. Obtained rank preserves from battle to battle.

Care about your soldiers
As said above - you can make your army consisting all of the elite guard ranked units. When unit suffers heavy losses it disbands, soldiers are routing away in panic and you lost this highly ranked force. If you see in a battle that your unit suffers high loss in a number of soldiers (almost half) - move it out from the battlefield somewhere to the rear or at least behind the lines of your more stronger units.

Fight besides the roads
It is useful to send reinforcements quickly from one sector to another (like, in one place you've got a victory, but in other, there is still harsh battle). Also, by road, you can rapidly evacuate waved and shaken units.

By any price, make enemy artillery out of order.

Use the pause to make orders
By this, you can evaluate the battle situation, oversee map, recognize threats and reduce losses. Hit `Pause/Break` button above `Page Up` btn on your keyboard, or if you're on laptop hit `minus` multiple times to slower game speed (and then `plus` to return back to normal). It's not a cheat, battalions in real armies had own commanders with own initiative and right to act, but in this game units always wait only for your command and rely only on your will.