From 21 : 30 until shortly after midnight the NK 9th Division crossed the Naktong at a number of places and climbed the hills quietly toward the 9th Infantry river line positions . Then , when the artillery barrage preparation lifted , the North Korean infantry were in position to launch their assaults . These began in the northern part of the regimental sector and quickly spread southward . At each crossing site the North Koreans would overwhelm local UN defenders before building pontoon bridges for their vehicles and armor .