The Six Years' War was a conflict fought a generation before the Derlavaian War. Algarve fought against an alliance consisting of Unkerlant, Jelgava, Valmiera, and Sibiu. Lagoas joined the anti-Algarvian coalition toward the end of the war, assisting Valmiera in halting a final Algarvian offensive aimed at the Valmieran capital of Priekule.
The war proved long and costly for all nations involved. In the end, Algarve was defeated and forced to agree to the Treaty of Tortush, a very harsh peace imposed by its enemies. Territorial losses included the Marquisate of Rivaroli (annexed to Valmiera), the newly-created Duchy of Bari under Duke Alardo, and the resurrected Kingdom of Forthweg which incorporated parts of northwestern Algarve.
Algarvians, including Mezentio, Crown Prince of Algarve at the time, would brood over the deep humiliation to which their kingdom had been subjected after the war, and were determined that one day Algarve would take vengeance upon its enemies. This desire for revenge manifested itself in the subsequent Derlavaian War a generation later.
Despite being on the winning side, Unkerlant underwent its own trials. In the chaotic aftermath of the war, its Forthwegian and Zuwayzan populations reclaimed their independence, partly because Unkerlant itself was riven by the Twinkings War between the rival heirs to the throne. The civil war wreaked a tremendous amount of damage on Unkerlant, leaving it weak for many years to come.
Within the Darkness series, the Six Years' War serves as an analogue to World War I, with a number of differences. Chief among them is obviously the war's length (six years as opposed to four) and the fact that Lagoas, the series' analogue to Britain, intervenes late in the war much as the United States did in the real-life First World War.