Great Turf Wars
There were already Turf Wars dating back to 2,000 years before the current setting, perhaps either as a competitive sport or a method to settle conflicts. The first recorded Turf War occurred during a period in Inkling history with many similarities to Ancient Egypt, roughly 2,000 years before the events of Splatoon began. However, as these Turf Wars of nineteen centuries later involved the Inklings and Octarians (instead of just teams of four Inklings), it has expanded into a bigger campaign, great enough to become a historical event.
Before the Great Turf Wars, the Inklings and Octarians had "amicable relations", until the rising sea levels caused them to lash out against each other in a power struggle over the remaining land.
As time progressed, the Octarians separated themselves from the other species of the Mollusc Era. They became dissatisfied with the amount of available dry land and waged war on the Inklings, the other dominant species at the time, to gain total control of the turf.
The Octarians constructed a variety of large machines known as the Great Octoweapons and began attacking Inkling cities.
The first battles of the Great War ended in victory for the Octarian forces. The diligent Octarians easily dominated the Inklings, who were unable to wake up early enough in the morning to defend themselves.(The Inklings have long led a hedonistic style of life).
Heralded by loud explosions, the Great Octoweapons quickly stormed the Inklings' central stronghold. Victory for the Octarians seemed all but certain, but due to a plug being carelessly pulled from its socket, their hopes were dashed. Lady Luck shone down on the Inklings, and the Inklings' victory over the Octarians was most likely due to their superior number of limbs. The Inklings secured a victory in the war and forced the Octarians to retreat to underground lairs such as Octo Valley, which were constructed from the ruins of the subterranean cities built by the remnants of humanity.
The Inkling faction had a fighting force known as the Squidbeak Splatoon led by Cap'n Cuttlefish, which fought against the Octarians during the war..