Great Eulhae War

Have Fusen build this big Empire by invading stuff maybe in a WW1-type event during the 1910s and 20s, then immediately decline by the 1930s because of whatever reason. After that in 1935, Fusen may do a Pearl Harbour to invade Yeoseo or something, prompting Jeongmi and Meisaan to invade it and its occupied lands, which ends quickly but is ferocious. This is the Sinju War. At the same time, Akitsukuni in the east is invading all of its neighbours to build a huge Akitsukunese/Fusenese Homeland that will stretch from Sinju to the eastern tip of Uju, which naturally also means they've been invading people's colonies and such. Following the victory in Fusen, the Allies then go and invade Akitsukuni to deal with this colony-stealing fucker who is later revealed to also be exterminating local populations. This is the Uju War.

Etymology
The Eulhae War started in 1935, with the year being 乙亥 (Eulhae) in the sexagenarian calendar cycle; this gave the name to the war. Actual hostilities started two months before the Eulhae year began, but it was only in the Eulhae year itself that the war truly became a global conflict. Historians at one time did not count the events that led to the actual fighting as part of the Eulhae War, but over time the name has become more symbolic than practical, and now represents the entire conflict from December 1934 to January 1941. 1935-02-04 (CNY 1935)