Answer
The answer is the Liaoshen Campaign and the Pingjin Campaign. During the decisive Liaoshen Campaign, Lin Biao successfully encircled and destroyed the bulk of the Nationalist forces in the Northeast. Following this victory, Lin Biao led his forces to participate in the Pingjin Campaign, which resulted in the capture of Beiping (now Beijing) and Tianjin, dealing a critical blow to the Nationalists.