Some Agents were already taking action to forestall outbreaks of rebellion. Captain John Nicholson, leading irregular cavalry based at Peshawar, seized the vital fort of Attock on the Indus River from its Sikh garrison while they were still unprepared, or undecided on rebellion. Nicholson's force then linked up with James Abbott's local Hazara levies to capture the Margalla Hills which separated Hazara from the other parts of the Punjab. When Chattar Singh openly rebelled in August, his force was unable to leave Hazara without fighting a battle. Although Chattar Singh twice succeeded in capturing the passes through the hills, he nevertheless failed to take advantage of this (possibly because of dissension among his senior officers and continual harassment by pro-British irregulars), and retreated into Hazara.