can you tell me one reason that led to it happening?
One of the reasons was Governor Evans' desire to hold title to the resource rich Denver-Boulder area.
Arapaho
 According to an historical narrative on the event titled "Chief Left Hand", by Margaret Coel, contributing factors that led to the massacre were: Governor Evans' desire to hold title to the resource rich Denver-Boulder area; government trust officials' avoidance of Chief Left Hand (a linguistically gifted Southern Arapaho chief), when executing a legal treaty that transferred title of the area away from Indian Trust; a local cavalry stretched thin by the demands of the Civil War; the hijacking of their supplies by a few stray Indian warriors who had lost respect for their chiefs and followers of Chief Left Hand (including a group of Cheyenne and Arapaho elders, a few well behaved warriors, and mostly women and children), who had received a message to report to Fort Lyon with the promise of safety and food at the Fort, or risk being considered "hostile" and ordered killed by the cavalry. (The tribe had been deprived of their normal wintering grounds in the Boulder area.)
what were the terms of the treaty of fort laramie why did it fall One of the terms was that  it established the Great Sioux Reservation including ownership of the Black Hills, and set aside additional lands as "unceded Indian territory" in areas of South Dakota, Wyoming, and Nebraska, and possibly Montana. It failed as neither parties fully honored the terms. who was this treaty signed by? The United States and the Oglala, Miniconjou, and Brulé bands of Lakota people, Yanktonai Dakota and Arapaho Nation. what is the language spoken by the last nation you mentioned? The Arapaho language. did this nation have enemies? Yes, the American troops. did any massacre also take place here? Yes, the Sand Creek Massacre.
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Arapaho [SEP] Sand Creek Massacre
In November 1864, a small village of Cheyenne and Arapaho became victims of the Sand Creek massacre, an attack by the Colorado militia, led by Colonel John Chivington. According to an historical narrative on the event titled "Chief Left Hand", by Margaret Coel, contributing factors that led to the massacre were: Governor Evans' desire to hold title to the resource rich Denver-Boulder area; government trust officials' avoidance of Chief Left Hand (a linguistically gifted Southern Arapaho chief), when executing a legal treaty that transferred title of the area away from Indian Trust; a local cavalry stretched thin by the demands of the Civil War; the hijacking of their supplies by a few stray Indian warriors who had lost respect for their chiefs and followers of Chief Left Hand (including a group of Cheyenne and Arapaho elders, a few well behaved warriors, and mostly women and children), who had received a message to report to Fort Lyon with the promise of safety and food at the Fort, or risk being considered "hostile" and ordered killed by the cavalry.