What is Manifest Destiny?
A fair bit of information on Manifest Destiny (MD) is available on the web site. It is not, however, always easy to dig through all that stuff, and I know that a lot of people don't actually go to the website and come straight here to the forums. So I will explain (for those of you who don't already know) what MD is.
The simplest explanation for MD is that it is a multiplayer turn based 4x game (exploration, expansion, extermination, and exploitation) which takes place across a time frame going from the dawn of history well into the future.
Play is organized by a host, who collects turn files from the players (generally via email). The host may also be a player in the game (there is no play advantage for the host as a player, they have no more access to play data than any other player does). Once all turns are submitted, the turn is executed, and all player's turns are processed simultaneously.
Play consists of founding cities, exploring the world, doing research in specific fields, improving cities (with structures), collecting world resources, handling espionage and counter espionage, fighting battles, designing and creating units, deploying and improving your world leaders, and handling diplomacy with your fellow players.
The map is randomly generated by a world generator, using parameters set up by the host. Once created, all players will play on this same world for the duration of the game.
You'll start by throwing rocks and sticks at each other, and end up wiping out each other's cities with antimatter bombs. Research is critical to the game in nearly every aspect. You research unit templates, components that go into units, city improvements, espionage tactics, resources (you won't ever see oil on the world map until you understand what it is). Keep up with the Jonses, or you will find your mud hut cities trampled by the other player's tanks.
Espionage will allow you to see what other players are doing, sabotauge their vital equipment, or steal technology. You will also handle counter espionage to keep the other players from doing the same to you.
All units in the game are custom designed by the players. You start with a template that allows you to add certain componets into certain slots. Filing in these components, you now have a blueprint for the unit, and you can create (through your cities) as many copies of this design as you need (or can afford).
Combat is not handled directly by the players, rather it is run during turn generation. The players will make sure they are on the winning end of combat by having superior technology, unit designs, and the appropriate battle orders for a given stack (a collection of units).
Leaders are central to the player's civilizations. They give bonuses to warfare and economic factors, and can be trained in cities using resources to be even more effective. Of course, you must have the technology available to train leaders in their fields of expertese.
There's quite a bit more to MD than that, but it gives you the basic idea. If you are interested, there is a plethora of information on the website to look through.