during the American Revolution, attempting to surrender West Point to the British and then later joining the side of the British.
Although a competent and capable General, Arnold's career was full of mishaps, problems and passed blame, despite fighting valiantly and vigerously for the cause, even sacrificing his own leg. Towards the end, he felt that his services were being overlooked while others were be given rewards which he believed should be his. This was his motivation for betraying the American forces.
Ironically, although providing him handsomely, the British never completely trusted Arnold, fearing he would betray them and return to the American's. As such he was provided with no major military tasks for the remainder of the war, and was later given a minor role in England.
Following the war, he lived out the rest of his life in England, save a short period where he lived in Canada. He died mostly unknown, having never really ammounted to much. According to legend, his last words were "Let me die in the old uniform in which I fought my battles for freedom, May God forgive me for putting on another."