A world without war isn’t realistic. We can invent these while conjuring a world history or while working in a given sovereign power’s file. Doing this requires some idea of the governments existing in those powers and their locations; otherwise it’s hard to know why the war is being fought, not to mention where, or what impact it has on the powers or anyone else caught in the conflict. It helps to understand technological or supernatural level of the combatants.
Is there a goal we hope this war will achieve? Perhaps we want simmering resentments between characters of today. Maybe we want a disaster to result as the logical conclusion of animosities we’ve set in motion. Is a resource under contention? Is that place a danger that one power wants to secure but which another wants to utilize?
Remember that former enemies can become allies later, sometimes through government upheaval, defeat, or just the passing of time. We’ve seen this on Earth, where in as little as a few generations, animosities have given way to mutual aid and reconciliation. Don’t be afraid to decide two friendly nations today weren’t enemies as little as decades ago. The speed of change may depend on technology and information; worlds with less, as in fantasy, might harbor animosity for far longer.
“Ethnic cleansing”, one group trying to eradicate another, has caused some of the worst wars; this sort of animosity won’t quickly disappear. A power might try to unite ethnic groups that have become split across nations. For example, Russia recently invaded Ukraine with one justification being that the Ukraine has regions with mostly ethnic Russians, who want to be part of Russia. There were also natural resources at stake.
Pride is yet another reason for war, as a new dictator must make a show of power. Or the dictator feels pride in their nation and heritage and feel they and their people have been oppressed or wrongly scaled back in a previous war. Starting a new war to right wrongs from a previous war is a classic. Leaders may have a grand vision of superiority or desire a fate different from, and better than, their current reality, for themselves or their kingdom, and the stress of this simmers for many years before leading to open war.
Taking back territory, resources, or something else believed to rightly belong to a kingdom is another reason for war. Sometimes these are perpetual disputes—until an empire absorbs both opponents.
We might want to indicate when a special military group was formed. Use this for famous knight orders or naval/space forces, even dragon riders, spectral groups, elite guards, horsemen, archers, or whichever groups you’ve created. If there’s a wizard or warrior order, like monks of a certain region, note their formation date, or when a powerful leader rose, influenced, disappeared, or fell. Did this group or someone from it accomplish something? Is there a force for good or evil, like the Justice League from comics? Cultures and Beyond (The Art of World Building, #3) goes into more details about groups we can create.
Creating magic items is one of the fun things about writing fantasy. For SF, legendary devices or ships are equally fun, particularly when they disappear, creating mystery, intrigue, and excitement when they show up in a story. These can fall under the protection of a sovereign power or individual and be contested. They can be feared or admired. They can come with a prophecy about the sort of person who might wield them one day, causing dreams and struggles. Each time you create an important item, add its invention date to the history, though lesser objects don’t need an origin date.
Missions to explore, rescue, kill or kidnap someone/something, or investigate strange phenomena can be listed in our history. Be sure to indicate the outcome, whether this is known to the characters or not. This history is for your files, so it’s okay to drop the truth in here and only reveal that to an audience in a story. Those who undertook a mission might be famous, giving us world figures. They might have also triggered an event, such as a war or new phenomenon, or discovered a new item or material.