Diplomacy is a game mechanic introduced in Emperor: Rise of the Middle Kingdom
To complete many missions within the campaign, the player will need to practice diplomacy. This will almost certainly include the sending of profitable goods towards other cities, whether they be neighbours or potential enemy threats to the country. The use of military force can upset or even anger relations between cities: therefore, it is important to use diplomacy where possible to win a mission.
There are some options that involve diplomacy.
Opening Trade Edit
To open trade with a trading partner requires that the city has a high opinion of you. In some missions, a city may automatically swear allegiance to you or already have a high opinion: however, most neighbours will only be "Apathetic" towards you and they will not entertain most player requests such as opening trade.
To increase a neighbour's opinion, the player should send goods to cities that they "need". Goods they "need" should be seen as the cities "export" options, aka cities that will buy goods from you and provide you cash to improve your city or reach a mission goal. Some goods increase favour more than others: as a general rule of thumb, the more money that a good provides the city, the higher the favour boost provided by the neighbour. However, if only a small gift is sent, then don't expect their favour to increase.
A city might send requests to your city: fulfilling these on time will boost the cities favour and likely result in cities opening trade.
For example, say a city sold and purchased the following goods in this table.
If you send gifts of items that they don't purchase from you, their favour isn't likely to increase, if at all. Sending them gifts of food (in this example, the city purchases Wheat) will also not increase their favour very much either, as food costs little money and may not serve much purpose. However, if you send them gifts for expensive items (in this example, the city purchases Silk and Lacquerware), their mood will increase. A large gift of an expensive good is almost certainly going to increase their mood substantially, to the point a city may just automatically open trade or ally without being requested at all.
Several missions within the campaign (the most notable are trading outposts created in the outer deserts during the Sui-Tang Dynasty) require that the player establish trading relations with a fixed number of cities. Note that you don't actually need to be importing or exporting anything to cities to meet this goal: it is only required that the game registers them as a "trade partner" to meet the goal.
Alliances are an important part of maintaining good relations with other cities. However, to gain an alliance with another city, your favour with the neighbours must be very high.
Just like with trade, the player needs to increase their neighbour's opinion, by sending gifts or fulfilling requests. However, an alliance is only going to work when the city "admires" you and won't work below this. It is possible to simply not send requests to ally with a city, as a city automatically sends an alliance request when they become "Docile", which occurs after being in an "Admiring" mood for some time. A city who is allied with you has the opportunity to become "Philanthropic", the highest level of positive favour possible in the game.
Some campaign missions require you to have alliances with a small number of cities. Alliances can sometimes be made even easier in some missions if several neighbours already have a high opinion of you from the start.
Some dynasties prevent you breaking alliances. Such dynasties usually don't permit alliances to be broken if no campaign mission in that dynasty requires you to "conquer" other cities as a mission goal. You can tell if alliances can be broken or not if the "Break Alliance" button is lit on the Empire Map, after selecting a city.
While not exactly a diplomatic method, some missions require you to conquer other cities using military force, turning the leaders of those cities into vassals. A city who has been conquered by your armies will be more likely to rebel and invade you if you displease them: therefore, it is important to practice diplomacy to quell rebellions. Vassals only rebel if you have no military strength, which usually happens if you send all your troops to fight a distant battle or to conquer another city.
In some missions, you may start with some vassals already. It is very important that you build some sort of military to keep these vassals happy, otherwise you may find it difficult to make money or be forced to give much of your early goods to these cities to keep them from invading your city.
Some missions also allow you to become vassal to an invading force. While you are vassal to an enemy, you are prevented from winning the mission and must build up your military substantially to defeat an invasion force to remove yourself from vassalhood: you will automatically lose the mission if you cannot defeat the second invasion while as a vassal. This can be useful in some missions with heavy military content if invasions come too quickly or are too strong. However, by going this route, be prepared to have to send a wide range of goods to your "leader" to keep them from getting upset.