Diplomacy Relationships
There are five diplomatic relationships between players: War, Cease-fire, Armistice, Peace, and Alliance. The first is the natural state, while the others can be achieved by signing diplomatic treaties.
During War, you can freely move your units inside enemy territory and attack their units and cities at will.
When two players decide to end hostilities between them, they can agree on a Cease-fire treaty. This prevents each player from attacking the other, but you can still move your units inside the other player's borders. After a set number of turns, the Cease-fire will lead back to War. Upon first contact with an AI player, it will automatically offer you a Cease-fire treaty.
If you wish for a more permanent peaceful coexistence with another player, you may sign a peace pact. This will enter a transitional Armistice state, which after another set number of turns will turn into a permanent Peace. Breaking an Armistice will drop you directly back to War.
At the moment two players enter into the diplomatic relationship of Peace, all military units belonging to either player that happen to be within the other's borders will be automatically disbanded according to the treaty. After this, you may not move military units into the other's territory until you either declare War, or forge an Alliance. Breaking a Peace treaty will drop you directly back to War.
An Alliance is the ultimate diplomatic relationship between two players. In this state, you may move units into each other's cities and your units may share the same tile; units no longer impose zones of control. However, alliance treaties come with obligations; you won't be able to ally with a player that is at war with a current ally unless you break the first treaty. If one of your allies declares war on another, the alliance with the aggressor is automatically broken. Breaking an Alliance will drop you to an Armistice treaty. An allied AI player will freely give you its world maps and shared vision, and will seriously consider trading technologies and cities, but in return will expect you to join its wars against other players.