 Behind the frontline there was a secondary defensive line before the strong PSS line for the Finnish Army to slow down the Soviet advance.The long-awaited Soviet offensive begun with overwhelming force and managed to push through Finnish defenses at the front line.Attack stalled at the PSS line but combined effort from assaulting troops and from naval infantry landing behind the Finnish lines made holding PSS impossible for the Finns who started withdrawing towards U line while delaying Soviet advance.