The War of the Lions was a Thyllanorian civil war in the 2nd century AC between two long-time rival noble houses -- the defenders and victors were House Karthmere, a house founded from descendants of Ethrykian immigrants to Thyllanor in the early First Era that came to power in northern Thyllanor and at one point purportedly was the dynasty on the throne when the fabled Thyllanorian kingdom was founded. They were recognized as the uncontested rulers of Thyllanor when the Dragomyr invasion began, and though they resisted conquest, they fell to subjugation and House Karthmere became vassals to the newfound Kingdom of Andoras, as a Realm.
The opponents and eventual losers were House Asmere, a closely related house that may have been a cadet branch of the Karthmeres at one time or another, but for reasons unknown came to be associated with the Alettonic people who once thrived across Thyllanor but were slowly being pushed out by the migration of the Edrossian tribesmen. Of the formerly many Alettonic kingdoms in Thyllanor, all of them were absorbed into the growing nation of the Tribe of Karth, one of which was the Old Kingdom of Caeiddfen -- later to be the inspiration for the rebel faction's own kingdom claimant, the Kingdom of Caeiddfen. House Asmere may have started out Edrossian if their story of origin is true, but their culture and ideals had been morphed within the mere span of decades to be as Alettonic as they came -- and this led to internal tension from the moment that Outhdian Karth'maere set foot on the beach of present-day Faercrest.
Closer to the time of the war, however, other factors had led to civil war breaking out. The death of Richard II Karthmere, a highlord known for his kind interactions with the Asmeres -- indeed, he married off two of his daughters to Asmere lords -- saw the throne being taken by Daemis III, a young man with idealistic nationalism embedded in his head. He found the Asmeres to be daft, and ordered them to renounce their Alettonic traditions and return to the proper Thyllanorian way of life. By the time of the Dragomyr invasion of Andoras, the Alettonic people were a dying breed -- the Sigorians, also Edrossian emigrants, came to inhabit and conquer a good portion of the Kingsland, and the Karthmeric Thyllanorians quickly spread throughout Thyllanor in the same way. Their ancestral lands continued to be reduced until all that remained was southern Thyllanor in regions such as the Duchy of Farrowhalt and Morano. In the Kingsland, they were confined to the northwest, near Brand, where a small population continued on fleetingly but diligently. It is also important to note that many of those who were ethnically Aletton or descended from the Alettons were also assimilated, as Daemis III attempted to do; some simply left behind their old ways and became, for all intents and purposes, Thyllanorian.Though Alettons were widely regarded as hard workers and kind people to those who knew them, they had long been seen as a minority in Thyllanor, and this was met with widely varying results -- some felt racist towards them, others went out of their way to aid them due to their status. The majority of Karthmere rulers and local lords felt it unnecessary to address the Alettons in a different way, and as such those who were lucky enough to be vassals of a considerate or neutral lord went on with their lives in stable yet limiting stagnation, while those who were affected by Thyllanorian nationalists fared much worse.
As both the acclaimed close relatives of House Karthmere and also culturally Alettonic, one would not be surprised to learn that House Asmere and the vast majority of Alettons were highly insulted by Daemis's implications to 'convert' them to their ideas of proper ideology and lifestyle. The patriarch of House Asmere, Aelfrud Asmere, sent a threatening letter to Daemis III, warning him that he should avoid meddling in the affairs of the Alettons lest they rise up and overthrow the government.
The war at the time had many different names -- the Asmeres would have called it a war of aggression against them. The Thyllanorians would have considered it a civil war. Historians of today, however, prefer a third way which negates any conflict and connotation of support for any side, as calling it a war of Thyllanorian aggression would of course imply their support for the Asmeres, which at one point was actually illegal within Thyllanor (and is still met with wary eyes, especially among the educated). It was noted that both House Karthmere and Asmere have lions on their house sigils -- House Karthmere is notable for embracing the color red in any form, while House Asmere was notable for dark shades such as grey and black. To form a solid compromise, it was instead decided to call it the War of the Lions because of their similar icons, and it also helps to imply that it was more of a war between estranged brothers and not two entirely different countries.