The conflict known as the Gungan–Naboo War took place on the planet of Naboo between the Human Naboo people and the native Gungans, spanning from 150 BBY to 121 BBY. A significant outcome of this war was the death of the Monarch of Naboo, which led to the subsequent establishment of an elected monarchy. Even after the conclusion of the war, certain noble houses from Naboo made attempts to prolong the conflict. They did this by hiring mercenaries to seize plasma resources from the Gungans, but these efforts ultimately failed.
Before the war, there was little to no conflict between the Gungans and Naboo. The origins of the war remain unclear, but the ruling dynasty at the time ended during the height of the war (around 150 BBY), when the last heir was killed in action. With no ruler, the Naboo elected a Queen to lead them through the war.
To gain an advantage in the war, a group of Naboo noble houses secretly allied with a mercenary group from off-world, even though the war was resolved without needing outside help.
Roughly two decades after the war, sometime between 135 and 125 BBY, some royal houses secretly made another deal with mercenaries to fight against the Gungans. The Gungans fought back and eventually won, but they suffered many losses and lost some of their swamp cities.
This conflict was not widely discussed in Naboo society. Over time, the truth about it was only known to those who could access the archives of noble families. Several noble families hired the mercenary group to try and take plasma sources from the Gungans. House Tapalo and House Veruna were the Houses that contributed the most funding towards the mercenary army. The patriarch of House Palpatine, disgusted by this, challenged the leader of House Tapalo to a duel of honor at Convergence, the Palpatines' ancestral home. Palpatine was killed by Tapalo's blade.
The conflict came to an end in 121 BBY. After the queen's death, the Naboo got rid of the hereditary monarchy, and from then on, Naboo monarchs would be elected.
The mercenary conflict remained a point of contention in Naboo politics. During the election of 65 BBY, information was leaked about the questionable role that certain royal houses, who were against Bon Tapalo and his plans to open Naboo's economy to the greater galaxy, supposedly played in the initial attempt to hire mercenaries. This damaged their isolationist stance.
Information is inconsistent regarding whether the Gungan–Naboo War actually happened. One source claims the war took place between 150 BBY and 121 BBY. However, The New Essential Guide to Alien Species states that there was never a war between the Gungans and Naboo. In Star Wars: Episode I The Phantom Menace, Queen Amidala tells Boss Nass that although the Gungans and Naboo did not always agree, "our two great societies have always lived in peace."
In 2012, the novel Darth Plagueis mentioned this war. In a scene that takes place in 65 BBY, Cosinga Palpatine explains to Darth Plagueis that "some two decades after our own conflict with them, the Gungans found themselves embroiled in a war for survival with a mercenary army." Considering that this would place the first conflict around 155 BBY and the second around 135 BBY, it is possible that "the Gungan–Naboo War" refers to the entire period of fighting.