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 ascending the throne of the Baharuth Empire, Emperor Jircniv'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 pattern: 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 viewed the Annual Wars nothing more than a "parade". However, the Empire's true aim with these skirmishes was to slowly weaken the Kingdom while waiting for an opportunity to conquer them with minimal losses. While the Empire used career soldiers, the Kingdom relied to drafties. Thus, by invading during harvest seasons, famers would be separated from their fields, causing crop failures that would eventually result in famine. 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 apologise 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 realised 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, it 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. After the Demonic Disturbance, the Kingdom was now determined to strike a decisive blow against the Empire and send an army of 245,000 strong to the Katze Plains while Crown Prince Barbro led a smaller force of 5,000 strong to Carne Village to gather information on Ainz.

Third Battle of Carne Village
On the way to Carne Village, Crown Prince Barbro hatched a plan to use the villagers as hostages, which would force Ainz to either retreat, robbing the Empire of their Casus Belli, or kill them and make him look like a tyrant.

Upon arriving, however, the Kingdom's forces were halted by the village's fortifications and the villagers refused to open the main gate as they needed time to hide the village's demi-human inhabitants. In addition, they were also unwilling to believe that Ainz was their enemy. Eventually, the Prince's patience ran out and he started threatening the villagers. This, in turn, caused the villagers to unanimously decide to join Ainz and rebel. They then opened the main gate to seemingly allow the Royal Army through, only for the village's Ogre inhabitants to kill them once they got close. Furious at this act of defiance, Barbro ordered the village be massacred.

While the Goblin Troop, Orges and elder villagers held off the Royal Army at the main gate, a second group escorted Village Chief Enri and the children to safety through the back enterance. However, they were intercepted by a Royal Army unit doing patrol, prompting Enri to use the Horn of the Goblin General Ainz gave her to summon a few more Goblins. Instead, she ended up summoning a whole army. The Goblin Army then engaged the Prince's forces and easily overwhelmed them and forced them to retreat.

Afterwards, Lupusregina Beta informed Ainz about the battle and he ordered her to massacre Crown Prince Barbro's remaining forces, having deemed him to be of no use, which she succesfully did.

Katze Plains Massacre
When Royal and Imperial Armies confronted each other at Katze Plains something immediately seemed off; the Imperial Army did not attack. As the Royal Army assuming the role of aggressor proved to be disasterous during previous Annual Wars, this effectively locked both armies in a stalemate.

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.