Annual Wars

Annual Wars were a series of yearly border skirmishes between the Re-Estize Kingdom and the Baharuth Empire designed by the Empire to slowly weaken the Kingdom in preparation for a full-scale invasion. Until the Sorcerer Kingdom joined the conflict on the side of the Empire after the Kingdom refused the recognize their legitimacy, turning it into an all-out war.

Background
After becoming Emperor of the Baharuth Empire, Jircniv Rune Farlord El Nix's main goal was to create a great empire that would survive the test of time. To this end, he set out to conquer the neighboring Re-Estize Kingdom.

However, rather than declare all-out war on the Kingdom, the Empire instead chose to engage them in small territorial skirmishes at the Katze Plains every year. These skirmishes mostly followed the same patern: The Imperial Army would engage the Royal Army, there would to a short scuffle and then the Imperial Army would retreat. Because of this, the Kingdom's nobles came to see the Annual Wars nothing more than a "parade". However, the Empire's true aim with these skirmishes was to cause food shortages. Unlike the Empire, which used career soldiers, the Kingdom relied to drafted peasants. Thus, by staging the Annual Wars during harvest seasons, the Empire could slowly weaken the Kingdom while waiting for an opportunity to conquer them with minimal losses. By the time the Kingdom realized this, it had already fallen into decline.

Following the Workers Invasion, Emperor Jircniv was forced to travel to Nazarick and apologize to its ruler, Ainz Ooal Gown. Upon arriving, he was horrified to learn to Ainz was an undead and that the tomb was inhabited by all kinds of monsters. Hoping avoid war, the Emperor proposed an alliance to aid Nazarick claim the city of E-Rantel, which Ainz accepted. Afterwards, the Emperor realized that Ainz had caused the Workers Invasion to arrange this meeting. Knowing the Empire couldn't defeat Nazarick alone, the Emperor began making plans to ally all human nations against it. To show the other nations of the danger Ainz posed as well as keep his promise to him and get an estimate on the extend of his power, the Emperor intended to have him fight during the Annual War.

While the Emperor believed Ainz was the mastermind, this was actually Demiuge's plan to declare Nazarick's existence to the world as an independent nation; Ainz was merely following it. However, Ainz had not been informed that the goal was to declare war on the Empire, causing him to agree to the Emperor's alliance. While the Floor Guardians were initially confused as to their master's intentions, Demiurge assumed he was using the Empire to gain legitimacy and quickly convinced his fellows guardians of the advantages of this.

A short while later, envoy was send to the Kingdom, demanding they cede E-Rantel to the newly founded Sorcerer Kingdom. However, the Kingdom dismissed it as just another excuse for war and declared war on both the Sorcerer Kingdom and the Empire. Following the Demonic Disturbance, the Kingdom was determined to strike a decisive blow against the Empire. Thus, three months later, the Kingdom send an army of 245,000 strong to the Katze Plains while Crown Prince Barbro led a smaller force to Carne Village to gather intel on Ainz.

Aftermath
Supposedly the Annual Wars came to a final end with a clear victory towards the Sorcerer Kingdom And Baharuth Empire. Devastating the Royal Army and its soldiers morale, the Kingdom was forced to secede the city of E-Rantel and its surrounding lands to the Sorcerer King. The fortress city then began the foundations of the nascent city-state of the Sorcerer Kingdom.