Hoping to repeat the success of the earlier First Phase Campaign Military_operation , the PVA 13th Army first launched Process_start a series of surprise attacks Attack along the Ch'ongch'on River Valley on the night of November 25 , 1950 at the western half of the Second Phase Campaign Military_operation ( ) , effectively destroying Destroying the Eighth United States Army 's right flank while allowing Preventing_or_letting PVA forces to move Self_motion rapidly into UN rear areas .