Whether a sovereign power is physically isolated (like an island) or borders other nations, it will have allies and enemies who shape the past and present due to tensions. In the early stages of building a world, it’s best to focus on the high-level aspects of these relationships, such as who is friend or foe. If we don’t have a reason for our decisions now, we can add it later. There are some basic reasons we’ll cover, however.
Ethnic hatred and other forms of racism are an unfortunate reality. We can pretend they don’t exist in our world, but if so, that’s best done by never commenting on it, not explicitly stating it doesn’t happen, as this strains credibility. If we choose to invent this sort of tension, understanding the source helps. Generally, unfavorable attributes are assigned to a group of people who often share physical traits that make them more easily identified on sight; this is part of where the superficial aspect of this enters. Don’t use only skin color for this, as facial features like eyes and noses can be used, too, such as how Asian and Jewish features have been on Earth.
The source of that unfavorable trait can be cultural, ideological, or based on government. For example, people living in an authoritative state might despise people in a relatively free democracy. This can be jealousy but is more likely to be propaganda put forth by that authoritative state condemning democracy and vilifying various aspects of it. Those in the democratic power might have similar negative views in return. Even if these governments change in time, the character assassination will linger (on both sides). These ideas are attached to the people.
In practice, borders change and people move, resulting in a mix of ethnicity that leads to conflict. This can cause one kingdom to attempt exterminating or expelling those of a different ethnicity. While we see this with humans, it can be done with other species, like elves against elves, or between species, like elves expelling humans from conquered territory.
This sort of expulsion can lead to perpetual war, particularly if cherished ground lies in contention. Jerusalem comes to mind. Retaliation can linger so long that each is convinced the other struck first or that they can never let an action go unanswered. The original reason for conflict can be lost to antiquity, which gives us some leeway to invent a conflict and a current claim by each side that justifies its stance, without worrying about having it “right.”
We might feel that some of our species and powers are more enlightened than this, and they may well be, but racism can still run deep and be hard to eradicate.
Natural resources can cause tension when sovereign powers can’t share them peacefully. This can happen when one power has the resource entirely within its territory or if the resource spans both powers. Newly discovered resources might cause a struggle to possess it. Weaker powers might also control a resource only to be threatened by a stronger one. While Earth-like resources are fine, materials of our invention can include items needed to power space ships and weapons, cast spells, heal people, or anything we imagine. Rarer resources are more valuable and we can decide what’s common on our invented world, but try to avoid making gold, for example, largely worthless, and something that’s cheap on Earth be prized on your world; this tends to confuse and it might be better to just create a new resource than to do this.
There are many reasons territory can cause tension. This includes resources located there, ethnicity (discussed above), religious significance, and the access the territory provides to other destinations. In the latter case, a landlocked power might wish to conquer another to gain access to the ocean. Territory sought is typically held upon acquisition unless something prevents this, such as the conquering army losing so many of its troops in victory that it cannot hold what it has gained. It might also become vulnerable to threats from elsewhere. Such fears can keep an invasion at bay unless something tips the balance. It might also succumb to local disease to which it doesn’t have immunity.
Ideological differences can cause one power to want another’s destruction or to become allies against such threatening powers. These differences may be not only political, but cultural, religious, and moral. Some populations, or at least their government, want to impose their own way of life on others, and see differing ideologies as a threat. This is one reason the United States supports struggling countries and why Russia supports different ones. The resulting conflicts are proxy wars where two superpowers are fighting each other via smaller countries they seek to influence. This can make for strange allies.
In extreme cases, internal conflicts can lead to rebellion, civil war, or government collapse. These dramatic events are useful in creating not only history, but a current situation that embroils our characters. A story taking place during such events is often about those events, however, so if doing this, we will need to work out the details of the conflict. If we don’t want to focus on such extremes, then lesser tensions might be needed. These less dramatic issues can eventually reach the extremes we avoid until later, if at all. Don’t be afraid to set these stories in motion out of a desire not to deal with the result now.
The conflict often involves freedom, abuses by government, or basic needs not being met, such as infrastructure or economic failures. It can be ideological in that people have lost faith in a leader or what he stands for. These reasons often merge, such as when slavery led to the American Civil War. This was both ideological and economic, as the south’s economy depended on slave labor that the north wanted to abolish for ideological reasons.