In order to establish diplomatic relations with another player, either AI or human, you must first meet them. To meet another player entails having one of your units be adjacent to a city or unit of that player.
Once you have met another player, you can engage in diplomacy with them as follows:
- Go to reports -> players (or hit the F3 key)
- Select the player you wish to engage in diplomacy with
- Choose Player -> meet
- Using the menus, decide on what you want to give another player, or what you want the other player to give you (see pacts).
- If the player is an AI player, and they are not cooperating, you can usually find out why by trying to arrange a peace or alliance with them.
While you can arrange a meeting with the players you are in contact with, you can't have more than the most basic intelligence about their civilization without establishing an embassy either by pact or with a diplomat.
Once your advances provide you with diplomats, and later with spies, your can observe and manipulate other civilizations more subtly than is possible with simple military observation and intervention. Both units are fragile and must move cautiously — often under guard — to survive. Diplomats can attempt only one diplomatic act, whose success or failure brings them to their end, while spies are more hardy and often survive their mission.
One action requires your unit to approach an enemy unit and attempt to enter its square:
- Bribe Unit. In exchange for gold, an enemy unit may be persuaded to join your civilization.
All other actions require your diplomatic unit to reach an enemy city alive and attempt to enter it. Enemy diplomats or spies in the city will oppose any hostile action. Either your diplomat or the defending diplomat will die. If the defending diplomat dies, you lose one movement point and may try again. The actions available at an enemy city are:
- Make embassy. Both diplomats and spies always succeed when you ask them to establish an embassy, unless your spy has already been working against that empire in which case she may be recognized and executed. An embassy gives you permanent contact with that civilization. At any time your embassy can provide you with basic intelligence about their government, treasury, trade ratios, and technology. An embassy also allows you to propose pacts to the other player, as described below.
- Investigate city. Your unit will report the status of the city, what units and buildings are within, and what it is currently producing.
- Sabotage city. Your unit attempts to destroy either one of the city's buildings, or all of its work towards its current project. Diplomats will select their target randomly but spies can be directed towards a specifc goal. After successful sabotage a spy is returned to her home city.
- Steal Advance. Your agent will attempt to learn the secrets of one technology held by the other civilization that you lack. Only one advance can be stolen from a city by Diplomats; Spies can steal more than once, but encounters more resistance each time. Diplomats steal a random technology, while a spy lets you choose which advance to steal. A successful spy is returned to her home city.
- Incite city rebellion. In return for gold an enemy city will change allegiances and join your empire, bringing along all units that call it home. After inciting rebellion successfully a spy is instantly returned to her home city.
- Poison city water. Only the spy can commit this atrocity, which empties the city granary and kills one citizen.
Players can make informal requests, agreements, and threats across the in-game chat channel, but to transfer actual property they must arrange a pact (and of course AI players can only negotiate through a pact). This can be done while the players have contact or may be initiated by a player who already has an embassy with the other. The window this brings up lets each player build a list of items he is offering alongside a list of what he will receive in return. These can include:
- A copy of your current map — showing either all the terrain you have discovered, or just which squares are ocean and which are land without revealing specific features, or both.
- Technological advances you possess.
- Cities in your empire (except the city with your palace), which will take with them any units that call the city home.
- Gold from your treasury.
- Access to your vision — everything your cities and units can see as the game unfolds. Unlike the other items, this is a persistent ability, and must be cancelled in your player list when you no longer wish the other player to see what you are doing.
- An embassy.
- A cease-fire, peace or alliance treaty.
Only if both players indicate satisfaction is the pact concluded and the transfer of goods made.
The default state with any player.
From neutral state you can decide to declare war to a player with no reputation loss.
Neutral units impose zones of control and cannot be attacked.
Neutral cities can be incited to revolt, but under representative governments, repeated incitement can lead to senate disapproval.
Unlike most other diplomatic states, war is declared unilaterally by one player on another; thus, declaring war only requires that you have already met the player once or he has declared war on one of your allies.
Only the War state allows one to attack a unit or to take a city without inciting a revolt.
Enemy units impose zones of control. Cities can be incited to revolt with no disapproval from the senate.
Sometimes, war becomes too costly for both parties. Cease-fire can then be agreed, and after a certain number of turns the diplomatic state between the former enemies will become neutral.
If you decide to declare war again before the end of cease-fire, you will lose reputation unless you've been provoked. Under representative governments, the senate may disapprove which can lead to anarchy.
Units impose zones of control.
A peace treaty is a step towards alliance.
When at peace with a player, declaring war or even returning back to the neutral state will lower reputation and (depending on the government) the senate may disapprove of your acts unless you've been provoked.
Inciting a revolt on a peaceful city is a provocation.
Peaceful units impose zone of control and can't be attacked.
Alliance treaties come with obligations, and you won't be able to ally with a player that is at war with a current ally unless you break the first treaty, with the usual consequences. If one of your allies declares war on another, the alliance with the aggressor is automatically broken with no reputation loss.
Zones of control don't apply and allied units can enter the same tile (including city and transport). Thus, cities can't be bribed by a diplomat entering in them.