Fourth Wasteland-Eidel War

It's not entirely clear who started it, but the entire war finished in a duel on Wasteland ground. A certain Eideli Saint beat the Wasteland host's chosen champion in single combat, thereby technically gaining leadership of the Wastelands. She wanted nothing to do with this, and essentially just set up the Wastelands as an additional province of Eidel (Technically being governess of the province for the next six years, until her death).

Why hadn't this happened before?

The defeat of a Wastelander champion in single combat was completely unprecendented. Wastelanders were a culture of warriors, and the warlord of a host would be especially powerful both through their own merits, and the Celestial weight afforded to them by such an honor. Every single past war could technically have been ended if an Eideli champion managed to best the Wastelander, but this was simply unthinkable. Eidel has no class of exceptional fighters in the same way that regions like the Midlands do- they're all overall very close to baseline (in-world, not irl) human. Meanwhile, the Wastelands band around elder ogres and royal ghouls with the strength of lesser gods. That's simply not something a human can fight one on one. Enter Saint Arterie.

Ramifications

Despite the two nations' bloody history, once Eidel actually won, the Wastelanders were very open to the idea of joining the empire. Being children of Eid, they weren't shunned as a lesser people by the Eideli in the same way that other vassal states were; in fact, Eideli noble houses were scrambling to marry Wasteland xiphaem practitioners practically en-masse.
See Effects of Wasteland Unification for more information.