When Hal Jordan joined the Green Lantern Corps, Sinestro was assigned to be his instructor. Jordan was horrified at his new mentor's totalitarian methods, though Sinestro maintained that his iron-fisted rule was necessary to protect his people from alien forces. During his training, Jordan helped Sinestro repel an attempted invasion of Korugar by the alien warlords known as the Khunds. When Jordan called for help from the other Green Lanterns, Sinestro's dictatorship was exposed and he was forced to appear before the Guardians for punishment. Katma Tui, the leader of a Korugarian resistance movement who felt that Sinestro's "protection" kept her people from growing as a society through contact with other alien races, was recruited as his replacement in the Corps. Though Katma Tui eventually grew into one of the most respected Green Lanterns, she and the rest of Korugar initially resisted her appointment to the Corps; due to Sinestro's actions, Korugar had come to consider the symbol of the Green Lantern Corps an emblem of terror and oppression.