While Disenchantment might be different from Matt Groening's other creations, it isn't any less well written, and the first two parts of Bean, Elfo and Luci's adventures weren't without some mysteries that needed to be solved. However, while the first major mystery appeared in the show's debut episode and featured a conflict between the Ogres and Gnomes, it has yet to receive any sort of resolution.
At the beginning of the series, Elfo was still living in Elfwood. He was eking out an unsatisfying existence making candy and having a secret relationship with the daughter of Rulo, the King of Elfwood. However, Elfo eventually came to the realization that it was time to move on right around the time Rulo was trying to have him executed.
Almost immediately after leaving Elfwood behind, Elfo was picked off by an eagle. Presumably, the large bird of prey saw this little green fellow as an exceedingly easy meal, but what the eagle underestimated was Elfo's mouth. Elfo didn't stop talking and the eagle eventually got bored, which caused the bird to drop him from an incredible height.
When Elfo got to his feet, he quickly discovered that he was in the middle of an all-out war between the Gnomes and the Ogres. It would be easy to assume that the Ogres were dominating the battle, but the Gnomes gave it all they had. Naturally, Elfo was dragged into the conflict as the Ogres didn't really see a difference between him and the Gnomes.
The reason the war was happening has never been discussed, but something drastic must have occurred to drag two such disparate enemies into a sustained conflict that inflicted such significant losses on both sides. This also did not appear to be a one-off battle, coming across as being part of a much bigger issue.
After the battle, both the Ogres and the Gnomes have continued to have a presence in the series, meaning the war didn't fully eliminate either side. The Gnomes, with their blue shirts and red pointy hats, keep popping up in various moments throughout the series, while Ogres, on the other hand, have continued to grow in prominence, something that became particularly apparent when it was revealed that Elfo's mother was an Ogre.
As the series moves forward, the Ogre/Gnome war could rear its ugly head again. With Elfo being half-ogre and Gnomes always hanging around, there is a distinct possibility the conflict might be part of future stories. However, knowing Disenchantment, it's just as likely the war happened for no reason at all.