From Civilization V Wiki
City-States are the smaller political entities in Civilization V. They cannot win a game – they're not competing against you – but they can greatly assist or impede your progress towards victory. You can befriend City-States and gain a number of important benefits, you can ignore them and concentrate on bigger and more important foes, or you can conquer them and take their stuff. It's up to you.
Types of City States
There are three different "flavors" of city-states. Each can provide you with different benefits if you befriend or ally with them.
- A cultured city-state can help you improve your culture.
- A maritime city-state can provide food to your civilization.
- A militaristic city-state can provide units to your army.
Communications with City-States
In order to communicate with a city-state, you must find it first. When one of your units encounters a city-state, the city-state will tell you what type it is (see above), and it will often give you a gift of gold as well. (This is another good reason to explore the world!)
Once you have met, the city-state may periodically make contact with you to ask you to undertake "missions" (see below). If you want to get in touch with the city-state, you can click on the city itself, or you can go through the Diplomacy Panel.
Your relations with each city-state are measured by "Influence Points" (IPs). They usually start at zero and your actions can increase or decrease them (yes, they can go quite negative!). Your current IP level is noted on the city-state's banner.
Gold: The Gift that Keeps on Giving!
One of the most cost-effective ways to increase your IP total is to give a city-state gold. To give a city-state gold, click on the city-state itself and then pick the appropriate menu item.
Give them Units
You can also give a city-state units. One way to do this is to move the unit into the city-state's territory and then click on the "Give Unit" button in the unit's Action menu. (You can also give them units anywhere on the screen through the city-state Diplomacy screen.) It should be noted that gold usually is the better gift, unless the city-state is specifically requesting units.
If you don't do anything, your IPs will tend to revert to zero over time: if your IPs are positive, they'll reduce by a small amount each turn. If they're negative, they'll increase by a small amount each turn. (The exact amount can vary based on the City-State's personality.) So if you want to maintain positive relations with a city-state, you'll have to periodically complete a mission or give them a gift.
Note that you lose a handful of IPs per turn for each of your units "trespassing" in a city-state's territory. If you are friends with the city-state, you can move through its territory with no consequences.
You've so totally angered the city-state that it will never accept peace with you. This occurs if you've gobbled up too many of the city-states around you – the survivors will band together and try to wipe you off the planet. They simply won't deal with you any more: there's nothing to do here but fight 'em off.
While at war with a city-state, your influence will remain negative and they certainly won't give you any stuff. However, unless you're at permanent war or they're allied with one of your enemies, a city-state will always accept a peace deal.
The city-state doesn't especially like or hate you. You can give them gold or do missions to improve your IP level, or you can degrade your IP by trespassing and suchlike.
If you're "friends" with a city-state, the city-state will periodically give you gifts – a cultured city-state will give you culture; a maritime city-state will give you food; and a militaristic city-state will give you military units.
If you're allied with a city-state, you'll get a stronger version of the benefits of friendship. In addition, the city-state will give you all of their luxury and strategic resources. Only one civilization can be allied with a city-state at a time – if multiple are eligible, whichever has the highest IPs gets the position.
Periodically, a city-state may announce a "mission" – perhaps it's being plagued by barbarians, for example, or its people seek knowledge of Natural Wonders, or perhaps they're being attacked by another civilization and they seek allies. If you complete the mission before another civ does so, you'll earn Influence Points with the city-state.
War of the City-States
You can declare war on a city-state at any time. You can do so through the Diplomatic Panel or by ordering one of your units to attack a city-state's unit or city. You can offer peace to a city-state through the Diplomatic Panel or by clicking on the city. It's important to remember that if you attack too many city-states, many will declare war on you and you will not be able to make peace with them. This can be shockingly unpleasant if you're not ready for them.
Liberating a City-State
If another civ has captured a city-state and you capture it from them, you have the option to "liberate" that city-state. If you do so, you'll immediately get a huge bunch of IPs from that city-state. In addition, that city-state will always vote for you during "World Leader" elections (see below).
You win a Diplomatic Victory by winning an election for the position of World Leader once the UN is constructed. If going this route, remember that city-states will vote for whoever has the highest IP level, unless they have been "Liberated" by a civilization, in which case they'll vote for their liberator.