Busting the Ghost in the Machine

I can't even spell that word.

Diplomacy is system manipulation. The Diplomatic AI in Civ 5 is neither stupid nor brilliant. It acts in a sensible and predictable manner. If a rival power is weak and you are strong, it will attempt appeasement until the balance of power shifts or until it can form alliance with other rivals against you. If you are weak and it is strong, it will patronize you, bully you and try to destroy you.

The diplomatic AI may be put to several uses by the wily player. First, it serves as an alarm system for malevolent intent. A player who establishes diplomatic relations with a rival power will be presented with evidence of that civilization's current amicability* through diplomatic discourse. A once-friendly civilization may begin to make demands, cancel treaties, and deliver insults, all of which are signs of impending war. This knowledge grants the player a few precious turns of preparation time before being attacked.

Trade: Give and Take, Then Take Some More

This is usually a good idea.

Diplomacy also allows for the trade of Strategic and Luxury Resources. Since Civilization maps generally spread their myriad resources across vast distances, trade is one of the best ways to acquire the Luxuries Citizens demand to produce all-important Happiness. Trade can also supply access to war-necessary materials like Horses, Iron, and Oil, and spark the growth spurts generated by We Love the King Day.

Generally speaking, demanding even trades of one Resource for one Resource is a good idea. Trading away multiple Resources for a single Luxury is rarely worth it. If a civilization won't make an even trade with you, they likely don't want the Resource you're offering all that much. Likewise, acquiring a trade weighted in your favor usually only occurs in situations where you have such a overpowering Military presence that you could take what you want by force.

Research Agreements allow civilizations to combine Gold allowances to develop a random technology mutually enjoyed by both members of the Research team. These agreements are vital for rapid Technical progress and should be undergone whenever possible, as they represent one of the few methods available to accelerate Technological development.

Treaties, Privileges and Alliances

He'll change his tune over time.

Civilizations may Open Borders with one another, allowing Units to pass unmolested through one another's territory. Opening borders is usually worth the exchange, as it allow your scouts quicker access to geographic information and other civilizations' trade networks. Beware a suddenly dour civilization with which you share an open border. If you feel an attack is coming, position you Units accordingly and do not renew Open Borders when the treaty expires.

Told you so.

Civilizations can also form Defensive Pacts, declaring mutual defense in the case of rival invasion. These pacts guarantee an ally in war, but also commit you to conflict with the enemies of the cooperative state. They're a good idea if you have martial intentions or suspect imminent attack by a strong power.

Pacts of Secrecy are non-binding agreements between two powers to work against a third. They have two practical benefits: they inform you of who is plotting against who, and responding to them gives you a chance to either build or sour relations with other states.

Pacts of Cooperation are similar positive non-binding agreements which serve as a barometer of intentions for other powers and can lead to easier agreements in alliance and trade.

You may also request that a friendly power declare war on an unfriendly one alongside you, and you will likewise be asked by some powers to take part in such shenanigans. Properly underlaid with Pacts of secrecy, weak powers can use this option to combine their forces against strong powers. When approached with such a proposal, measure your response carefully. You may accept, refuse or request ten turns to prepare.

It is sometimes expedient to use the knowledge afforded by such a proposal to betray the party who approaches you. A nation preparing for war will often move its key Units to the frontier, leaving valuable real estate open to supposedly secure allied flanks. By moving quickly, you may be able to snatch territory during the ten-turn wait and take advantage of your co-conspirator's trust.

Bribery

Sometimes you have to swallow your pride and pay The Man. A bribe can carry diplomacy a long way. Gifts of Gold can smooth over stalled treaties. The AI often responds favorably to large infusions of cash or Resources as a means of inciting attack on other civilizations. Gifts of Gold can also be used to curry favor with powers who are souring toward you, but in the long run doing so is rarely worth it.

A much more powerful currency of bribery is a City. The Diplomatic AI regards Cities as highly valuable in negotiation. In a war you can't win, bending all your resources on capturing an enemy City and then bribing an opponent with it may buy peace for a time. When attacked by allies on multiple fronts, a gifted City may even convince your former attacker to betray their partner in crime. While the loss of these Cities sting, bribery is sometimes the best recourse in a no-win military situation.

Of much more interest to the would-be despot is how to get the AI to bribe YOU. The computer player can be manipulated into suing for peace and offering incentives as long as it has no reason to distrust your word, i.e. a history of broken treaties. Destroying several AI Units and capturing a city or two is often enough to provoke peace talks where the AI Leader offers large gifts of Gold, Resources and even Cities.

They are especially vulnerable to such manipulation when they are fighting an unwinnable multi-front war, so conceiving Alliances against them may further push them to desperate acts of gift-giving. Since captured Cities can be quite a pain, it's sometimes worth taking the bribe and respecting the treaty rather than going to the trouble of flattening a rival state.

Betrayal

Sooner or later, you'll find yourself in a situation where breaking an agreement is in your best interest. Don't shed a tear over this. Machines don't have feelings yet. Unless you fear the reciprocity of Skynet sometime in the future, embrace betrayal like a sultry lover.

Whenever possible, observe the forms when betraying an enemy. Allow treaties to expire before attacking, as future diplomacy with other civilizations may be soured by a perception that you are untrustworthy. Of course, if you don't give a damn what other civilizations think of you, then feel free to launch your very own Tet Offensive, ignoring cease fires with impunity and jumping unsuspecting Units in the open.

Diplomacy With City-States: The Cost of Friendship

Diplomacy with City-States can render great rewards. A friendly City-State may provide access to Culture Points, free Units or Food Bonuses. City-States are glad to provide Allies access to adjacent Resources, open their borders, and go to war against enemies of their patron.

City-State diplomacy is an exercise in catering. Bribes of Gold guarantee a degree of fidelity from City-States proportional to the amount spent. City-States also occasionally offer missions to the player. Fulfilling these missions grants large amounts of Influence. You may also gift Military Units to City-States, which grants a small amount of Influence.

Regardless of the initial source, City-State Influence erodes over time and must be constantly renewed by new gifts and mission fulfillment. Each time Gold is used to buy Influence, the amount of Influence that the same amount of Gold will purchase next time decreases slightly. Some Policies can modify the rate of this erosion in your favor, as can Alexander's Leader power.

City-States are relatively weak and tempting targets for conquest. Whether you choose to cooperate with City-States or conquer them, it is good to know that City-States look out for one another. If you capture two City-States, the remaining City-States will become worried, and from then on every City-State you conquer increases the chance that one or several others will declare Perpetual War on you. Cities which do so will remain hostile throughout the game until destroyed.

Missions

City-States reward you for doing their dirty work.

Missions for City-States are usually Military or construction oriented. A City-State may wish you to destroy a Barbarian encampment or a rival City-State. They may call on you to aid them in war with another power by destroying several Units controlled by that Civilization, or ask you for gifts of Units to combat their enemy.

They may request that you create a Great Person of some type. Just the birth of the Great Person will fulfill the mission. You do not have to grant them as a gift to the City-State, and the Great Person may be expended as normal. City-States may also ask you to construct a specific Wonder in one of your Cities to inspire their populace. Again, you retain all the benefits of the Wonder.