On the side of all the law abiding citizens of Middle Earth, protectors of the weak and defenders of peace are the 10 nations of the Free Peoples (FP). The FP nations start the game in most respects as much more powerful than the 10 Dark Servants (DS). Economically they are superior to all the DS team. They have some of the best characters and artefacts and some nations have incredibly powerful armies. The one thing against them is the way they are spread out on the map. Eight of the DS are combined in a small part of the map (in Mordor) and therefore have to co-ordinate much more. This engenders a better team spirit and improves co-ordination. The FP have more space, more powerful armies and more resources. But they don't communicate as well with their neighbours because they don't have to.
In the first 10 turns the FP should concentrate as much as possible on causing as much damage to the DS team. At game start they have a much stronger hand and should try to eliminate at least 2 of the DS nations. The Dragon Lord can be eliminated if the Dwarves, Sinda and Woodmen co-ordinate. The Witch-King can be eliminated if the Noldo, Arthedain and Cardolan work together. The Ice King is especially vulnerable to a combined Eothraim/North Gondor push. The Fire King capital can fall if the Sinda, North Gondor and South Gondor nations combine at the Ithil pass. You must dish out as much damage as you can in the early turns. If all 10 DS nations are intact after turns 10-15, you will have lost the initiative and will be on the defensive. The DS agents will then have the upper hand and will be out to get you!
The Dark Servants (DS) have the inferior position at the start. They are much weaker than the combined Free People (FP) nations. They have smaller armies and poorer economies. Despite this, if they can weather the initial onslaught then they have a good chance to win.
The 8 nations of Mordor must work together. They are packed together in a small space. Only co-ordination and team work will save them. If they can work as a team and remain intact until turns 10-15 then they should then be able to turn the tide of the war. Their agents (especially the Cloud Lords) will be able to assassinate at will. The armies of the FP will not be as effective, and in a character war so the DS are better. The general rule of the DS is work together as a team, co-ordinate with each other, support your 2 isolated team-mates (Witch-King and Dragon Lord) be nice to the Neutrals. Do this and you have a better chance of victory.
As well as the 20 predetermined sides there are the 5 neutrals. These nations hold the balance of power in Middle Earth. Each of them can have a profound effect on the outcome of the war.
An early declaration for the DS by Rhudaur will save the Witch-King and put the balance of power firmly into the DS side in Eriador. Conversely, the decision to declare for the Free People makes the Witch King's position extremely precarious.
It's usually better for the Dunlendings to declare for the FP but a combined Witch-King / Rhudaur / Dunlendings combination should be more than enough for the Cardolan / Arthedain nations.
The Corsairs and Haradwaith are incredibly powerful nations. If both these declare for the same side they will turn the tide of the war. It can be a lot of fun fighting each other though!
The Easterlings have Population Centres which are accessible to both the DS and the FP. They, of all the neutrals, must play a diplomatic game. If they can keep out of the war until turn 10 though, they can be decisive in the east.
Be nice to a neutral! Most people who play neutrals tend to be the diplomatic ones. They enjoy contact and discussion. They will usually join the team that talks to them the most, NOT the one that offers them the most. If you are open and honest with a neutral from the start, if you share with them your sides successes and defeats, then you will very likely get them on your side. NEVER THREATEN A NEUTRAL!!!