20 August 1967, Lieutenant Colonel Rai Singh, who was commanding a unit at border outpost at Nathu La, was ordered to construct a wire fence along the watershed at Nathu La after the first Chinese intrusion at North Shoulder. Against determined Chinese resistance and in spite of use of physical force by them he successfully completed the task. 

When on 7 September he was ordered to extend the wire fence along the watershed from North Shoulder to South Shoulder the Chinese fired a few shots. Bayonets and rifle butts were freely used and in the scuffle Lieutenant Colonel Rai Singh was injured. Undaunted he carried out his task and on 11 September when his men were strengthening the wire to prevent trespassing, the Chinese soldiers suddenly opened fire with artillery, mortars and recoilless guns. He retaliated with all available weapons. He himself opened LMG fire on the Chinese bunker and also gave covering fire to his own troops on the wire to allow them to get back.

When his own bunker was damaged, he came out in the open, and finding a wounded soldier unable to fire, picked up his weapon and engaged the Chinese. When a Browning machine gun commander had been killed, he manned it, but hardly he fired a few bursts when he was hit in the stomach and collapsed on the spot. Although he was hit in the head by a splinter when he was being evacuated, yet he continued to give directions and exhorted his men to keep on fighting.

In this action, Lieutenant Colonel Rai Singh displayed conspicuous gallantry and leadership of a high order.