Nickelodeon aired the first two books of The Legend of Korra in the United States. Book One: Air and Book Two: Spirits both originally premiered on Nickelodeon before later changes to the show’s distribution.
Book One aired on Saturday mornings from April 14 through June 23, 2012. It introduced Korra’s arrival in Republic City, her airbending training with Tenzin, the pro-bending arena, and the Equalist movement led by Amon.
Book Two began on Nickelodeon on September 13, 2013, moving to a Friday-evening schedule. It expanded the series’ mythology through the spirit world, the Southern Water Tribe conflict, and the origin story of Wan and Raava. Nicktoons was involved in later reruns and distribution, which can make it seem like the original network was different. The stored answer refers to the initial U.S. broadcaster for both books, making Nickelodeon the canonical answer.