The Badlands is a section of Mobius located between the Overlander territory and the Kingdom of Acorn's city of Mobotropolis, and was an area of hostility between the Overlanders and Mobians during the Great War.
The Hedgehog brothers Jules and Sir Charles located the escaped convict Julian Kintobor in the Badlands in a routine scouting mission during the Great War, where he was rescued and brought back to the Kingdom. (StH: #50, SSS: #2)
During the subsequent reign of Julian/Dr. Ivo Robotnik, the Badlands were used as a battleground for Sonic the Hedgehog's fight with a Beetle-Bot, Princess Sally's Soul-Quest with Nicole and the Knothole Freedom Fighters, and a path that Sonic took en route to the Robo-Hobo Jungle. (StH: #15, #40, IYF)
After Robotnik's death, Knuckles the Echidna somehow came across the Badlands while returning home from his adventures in Mercia and Albion. Locke and Sabre watched Knuckles confront a gang of bears on motorcycles, and were concerned by the now awakened use of Chaos powers, forcing a need for a meeting of the whole Brotherhood of Guardians. Knuckles used a Guiding Star Gem to return home from the Badlands. (KtE: #13, #14, #15)
In 3236, the Badlands was the battleground between a large SWATbot horde dispatched by the Eggman Empire, and the defenses at Fort Acorn consisting of the Mes Braves Battalion, Chaotix, Amy Rose and a G.U.N. expedition force with Rouge the Bat. Ultimately, the SWATbot forces were destroyed by the combined defenses, which ensured the force field around the irradiated Robotropolis was maintained. (StH: #131, #132, #133)