Tactics 2's main upgrade from one is the use of chain combos. Combos are the basis of the tactical system. This is all a bit disjointed, but here goes.
Let's cover armies first. Armies can have 1 Commander, 2 Strategists and up to 7 Officers depending on your Commander Leadership score. The Commander is the leader of the army ... if he is defeated you lose. The Strategists have two special abilities to use that I'll cover later on. And the officers are just more men to fight for you.
Each officer (this includes the Commander and Strategists) has 3 ability scores ... War, Intelligence, and Leadership. These abilities are what your tactics are based off of. Covering this in a little bit.
There's different types of units led by different officers. Guan Yu will lead a Horseman unit at the beginning of the game, Liu Bei will lead Infantry and Zhang Fei will lead Spearmen. These aren't the only generals in the game (far from it) but they're the ones you start out with, so I'll use them as the example. As your generals gain experience, they can upgrade to new units that may have better attack, better defense, different movement ranges, different missle ranges (for archers) or even different abilities (like the ability to construct catapults or archer towers for engineers). So Guan Yu can upgrade to Cavalry and Brigadiers, Liu Bei can upgrade to Rangers or Marines, and Zhang Fei can upgrade to Pikeman.
Guan Yu's Horsemen have great movement range on the plains, but poor range in forests and the like. Liu Bei's infantry can move well unimpeded through almost all terrains. Zhang Fei's spearmen are like Liu Bei's infantry but a little stronger ... when he upgrades to Pikemen, he becomes even stronger but loses some movement range, making it more difficult to utilize him.
Your units can always use the Attack option ... attacking does damage to your opponent and to you (though less to you). If you have a bow or crossbow unit, they can also use a ranged bow attack that does no damage to themselves and also lowers enemy morale.
BTW, morale is important because it determines initiative each round ... the higher the morale, the sooner you go.
Now, combos. Combos are built off of the different tactics in the game. A tactic is a move that can be used whenever the conditions for it are fulfilled on the battlefield. If a tactic is successfully utilized, then it is removed from character's options. You can regain tactics by defeating enemy officers and destroying enemy supply depots or by having your own supply depots restore them every round. The tactics power is based off of the officer's related ability score ... if you use a War Tactic with an officer who has a low War ability, it won't work well. However, using a War Tactic with an officer who has a high War ability will do exceptionally well. So don't go teaching your Leadership officers War tactics or your Intelligence officers leadership tactics. No real sense in it. Work with your officers strengths.
Tactics do not damage YOUR troops, so you want to use them more than the Attack option.
Tactics are available when certain requirements are met. For the Charge tactic, you simply have to be adjacent to the enemy and after the charge the enemy turns to face you. For the Pursue tactic, you have to be adjacent to the enemy with the enemy facing away. There are many many different tactics in the game, but I'll use these two for the example.
Say you have Unit A facing Enemy and adjacent to them ... and on the other side, one square away is Unit B. Unit A has charge. unit B has pursue. Unit A on his turn can use Charge to push the enemy one square away and turn them to face him. This pushes the enemy to be adjacent to B and they will now be facing away from B. Since B is now adjacent to the enemy and the Enemy is facing away, B's tactic Purse will trigger and B will attack as well. This is a Tactical Combo. The more tactics in a combo, the more experience your officers will get.
One of the abilities of the strategist is the ability to Link a tactic. I'll cover the other ability in a second or two, so bear with me. Suppose you have Strategist adjacent to Enemy A and Officer adjacent to Enemy B. On his turn, the Strategist Links the tactic Fire Attack to Enemy A. The next tactic used by your force will trigger the Strategist to use his tactic as well. This allows you to combo even with different units across the board. So if Officer uses Charge on Enemy B, then Strategist will also use Fire Attack on Enemy A.
There were 2 main new goodies added to Dynasty Tactics 2.
Chain combos can be ordered by your Strategists. The Strategist loses 5 morale to order every officer in his range (determined by Intelligence) to be able to do a Chain Combo. So if Officer is adjacent to Enemy and Officer is in range of Strategist's Order Chain command, Strategist can order the chain ... on his next turn, Officer can chain his tactics together to make a combo. So officer could chain a Charge and a Pierce together on his next turn to Charge the enemy and push them one square away and then follow through with a Pierce to break through their lines.
Another new goody is the Friendly Tactics. There are a lot of relationships between the different officers in the game and based on these relationships, you may sometimes trigger different tactics. Say Guan Yu and Zhang Fei (two brothers in the game) are on opposite sides of an Enemy. Both Guan Yu and Zhang Fei are set to Charge. Guan Yu charges, which triggers Zhang Fei's charge ... in addition, a new Tactic, Double Blade, is triggered because of the friendship between the two. If Liu Bei were involved as well, the tactic would become the Triple Strike, because all 3 are Oath Brothers. There are many other Friendly Tactics.
Hope that helps some. I'll clarify any questions you have as best I can.
