On 3 September 1879 Sir Pierre Cavagnari, the British Resident in Kabul, and his escort were massacred by mutinous Afghan troops, initiating the second phase of the Second Anglo-Afghan War. A force was assembled and named the Kabul Field Force, under the command of major-general Frederick Roberts. After defeating Afghan forces at Chariasab on 6 October, Roberts marched into Kabul on 13 October. At the end of November, an army under the command of Mohammed Jan Khan Wardak, who had denounced Yaqub Khan as a British puppet and instead declared Musa Jan the new amir, gathered in the area north of Kabul. On 11 December a small detachment (c.170 men) of the 9th Queen's Royal Lancers and the 14th Bengal Lancers encountered a 10,000+ Afghan army advancing on Kabul. As it was of the utmost importance that Mohammed Jan's advance was delayed the woefully outnumbered Lancers charged the Afghans. Heavy casualties were suffered and the Afghans continued their advance. On December 15, the Afghan army began to besiege the British forces entrenched in the Sherpur Cantonment.