For some more clarification on the mechanics of this game.
The phases will go in this loop, each loop being a year: (not including the year 0 build)
Movement (Spring)
Spring Retreat
Movement (Fall)
Fall Retreat
Builds
After the fall retreat, all supply centres that are currently occupied by an opposing unit switch sides. The supply centres are the countries with dots. (The territories people start with)
It's important noting that if someone is on your supply centre at the end of the spring phase, it's still yours because it doesn't change hands until the end of the fall phase.
The amount of supply centres you own at the end of each year is the max amount of units you can have. If you have more units than supply centres, you are forced to disband units on the build phase. Alternatively if you have more supply centres than units on the build phase, you will be allowed to build new units on any of your open supply centres.
If you ever find yourself with zero supply centres, then you will not be allowed to have any units making it impossible to take any more eliminating yourself from the game.
There can only ever be one unit in each territory.
Each unit can do 1 of 4 things during a movement phase.
They can
hold
which means they don't move.
They can
move
to another territory.
They can
support
the action of another unit. This increases the priority of the units action.
Fleets can
convoy
armies across water.
This link gives visuals and explains it.
http://webdiplomacy.net/intro.php
All orders are processed simultaneously so there will be conflicts. For example, if two different armies try to enter the same territory with no support, they will both "bounce" back and fail to enter it. One of these armies would need more support than the other to be able to take it.
Finally, if a unit gets displaced (if an army with 2 support moves onto a territory with only 1 army, it will displace the 1 strength army) it will either have to retreat or disband in the retreat phase.
Disbanding it removes it from the board. Retreating it lets you move it to an OPEN neighbouring territory that wasn't bounced that turn.