- Include your Players:
Most players, be them roleplaying experts, writers, or brand new to the genre; will put some kind of effort into crafting the back story of their character in question. How deep that story is largely differs by player but each one might grant you the pleasure of enriching your world. Tie them in to your story or history using what they've given you. I had a Dragonborn Paladin who's city was attacked by undead. The DM used that to create an undead war and the BBEG leading it.
- Start small and let it grow:
The moment you create a concept and then bring it to life in a role playing game where other humans playing characters can interact with it? It becomes part of your living world and it WILL grow from that point onward. You might create a noble family today that one of your players has lineage to, and find in a week from now that player has helped you shape a story around that family; and now you are hit with the inspiration for a rival noble family.
- Wars are messy affairs that have lasting consequences:
Take your inspiration from Warcraft, Diablo, Starcraft (I enjoy Blizzard...don't judge) ; Lord of the Rings, many fantasy novels; you can almost always find a war that either holds the centerpiece of the novel or created the setting for it. If you are going to create a war, like the War of the Ancients, or the Scourge Invasion known as the Third War; start with a reason. One side was after something another side opposed. Follow up with a time frame and locations impacted / destroyed / altered by the war. Finish with how it concluded, if it's conclusion altered something major, and if any heroes of both sides stood out; be them dead or alive.
An example is the Third War of the Warcraft Timeline. The Paladin Arthas witnessed a travesty he felt he was capable of stopping. To save his people from an undead plague started by a cult working unknowingly for demon lords; he would use any means required. Including culling an entire town of infected villagers before they could be turned into undead. He became tainted by his otherwise noble cause and a sword through which a malevolent spirit drove him mad, then killed and raised as a pawn for the demons. He lead their undead army against several major cities that were decimated in the process and left as ruins to be rebuilt or taken over. Even the path in which his army took left behind a 'scar' that cut through the land and left it forever changed.
This is an extremely condensed version of events but includes the key suggestions outlined above. The history of the Third War was left behind by MANY people and factions; but it started with a single Paladin, a cult, and the demons behind a plague instrumental in bringing their ruler to the human world. Look what happened from there...