Although there's very little info available even in the wiki still, I'll try to give some tips based in my experience so far:
1. Conserve money. That includes retry a few battles in the beginning, when it's possible to win without being hit. After that, learn to use the weapons wisely (chart in manual, page 74) to deal critical damage while prevent yourself of being critically hit. After some time, you can find yourself without money if you're careless.
2. Balance is the key. Doesn't matter if one of your unit is well armed and/or have much infantry if the others are not. You can use that unit ONE time in your AND your opponent's turn. Try to rise infantry equally while giving each one the most efficient weapon/item. If you don't have units left anytime you're attacked, you lose. Also, keep note of your team "build" since each country likes one type of weapon - like Indians and their axes. If your only strong units are blade users, you're asking for lose.
3. Deal with one country at time if possible. Although the wiki says there's no consequences in skipping a turn, it forgot (now they added it) to mention that the declared enemy leader(s) will manage their territories and enemies will gain a slightly increase in infantry numbers EACH turn. I recommend, in order and one at time, deal with: China, Mongolia, India, Oceania Islands/Hawaii, Oceania, USA/Indian, Macedonia. I only played till there, but probably Babylonia is next.
4. Plan your actions. You can do two, then later three and, I assume, finally four actions per turn. However each action will require some skill points to start and units used to fill those points will NOT be available till your next turn. If you have a low number of units and/or the enemy is pretty strong, remember to save them for battles. I don't think events will ever expire so don't worry about this.
5. Counter-attacks and surprise attacks. Every and single nation will retaliate if you take or attack one of their bases. Counter attacks will happen during their turn and you will NOT be able to use units used previously in your turn so be careful: no units or lose this battle and they'll take their base back. Sometime after the Hawaii events, you'll have to deal with occasional pirate attacks (every two or three turns - add to it normal counter attacks and will be two attacks per turn!) 'till you finish USA/Indian - or at least I don't see them anymore. If you skip about three turns without doing anything, an Independent Nation (bandits) will attack. Since they're weak, it's a good opportunity to score some bonuses. (Great victory = +20 infantry for each alive unit that won the battle, while simple victory = +10).
6. Some places are insanely hard and/or plainly impossible to deal at some point. Just after unify Japan, it's IMPOSSIBLE attack Russia. Do NOT declare war to them and save or you'll have to deal with 1XX turns powered enemies - and they already start with 3000~4000 infantry each while you'll LOSE half of yours in the first battle (and later ones too if you don't have Archimedes, I think, in your team). Same applies to south Hawaiian island, although I managed to conquer it with my 2000ish team.
7. Take a turn or two to complete events. Some of them will reward you with H-Scenes, Items, Units and money - very useful. And yes, to have Billy the Kid (permanently) and Columbus in your team, you have to pay 80,000. But they worth it, oh, they worth it (Billy for fun factor and Columbus for awesomeness).