So why do civilizations tend to interact with one another? Resources of course! Brazil might want to trade silver, a resource you do not have, for your supply of sugar. Now there is trade and relations established between these civilizations. Once trade is established, the countries are connected. Culture and resources are exchanged, and eventually a friendship can develop. A friendship does not necessarily mean a great relationship, but it can mean recognizing a tactical advantage of working together. The Mongols encouraged working with many groups of people, including Muslims. Sometimes tolerating and working with people can encourage the development of new technologies, which is exactly what Chinngas Khan desired. Working with other civilizations in wartime can have a massive advantage, as your influence expands along with their allies. Player in Civilization V will have to think twice about agitating you if you are allies with the civilization or city state at their border. Working together with another civilization can lead to a friendship, but more often than not, it will lead to a mutual trust and tactical advantage between players. This simple and pivotal concept is featured in Civilization V.
If there are so many great advantages to working together, why do people not do it? Contentions between civilizations can often develop around controlling resources and conflicting ideals. Just as they can be valuable, resources can sometimes be fiercely desired. India demands your supply of gold in exchange for nothing. If a military force is strong enough, there is sometimes simply nothing someone can do about giving in to demands. If resisted, the people can be harmed and potentially destroyed by the malicious, conflicting civilization. Barbarians often are a subject of discussion with valuable resources, as they will indiscriminately pillage lands for their gain. Barbarians, in history, have a tendency to pillage smaller civilizations, mostly religious buildings and churches, as they were often a source of wealth. Land is also a commonly contested resource. The Mongols display this hunger for land when they conquered multiple regions of Eurasia. Ideals and religion is one of the other major points in contention between nations. Simply disagreeing with a lifestyle can be the ultimate insult, especially when disagreeing with religion. With the wide interpretations of the Sharia code, many early Islamic groups attacked several different people and civilizations out of anger due to their different lifestyle choices. This can be seen in Civilization V, as other players may become wary if you continue to spread your religion to their regions or allies. A player can also garner negative attention if they continue to facilitate positive relations with city-states and their enemies. Land is a valuable resource that can be contested if you expand your empire quickly. If you expand your land near another civilization, their leader may become angry, as you have taken the land that they could have used. Relations can truly be nasty and aggressive in history and Civilization V.
Can other civilizations be beneficial or detrimental to your civilization? History has shown many instances of cooperation and warfare that resulted in the interaction between civilizations. Civilization V has a historically accurate display of civilization interaction. While some of it may be simplified, the overarching themes have been captured in the game for the players.