The YSE/Elysia Treaty of YE 30 is a military and economic treaty between the Elysian Celestial Empire and the Yamatai Star Empire that effectively ended tension between the two nations and created a rock-solid military alliance that continues to this day.
Yamatai and Elysia had a history of warfare. After four wars, the two nations were sick of fighting one another. Elysia was finally gaining its economic footing and military power, and Yamatai's new Empress saw them as a valuable potential ally against a growing anti-Yamatai alliance. By creating this treaty, the YSE/ECE conflicts of the past would be replaced with brotherhood, and both Elysia and Yamatai were able to reduce threats against their nations.