In 1967, with the rank of Lt Col, he was commanding the 2 Grenadiers battalion deployed at Nathu La, Sikkim, when the Chinese Army attempted incursions into Indian held territory, leading to the Nathu La and Cho La clashes. In order to hold on to the Nathu La pass and defeat the Chinese attempted incursions, Lt Col Rai Singh led his men from the front and displayed conspicuous bravery and exceptional leadership in the face of the enemy.