The war had begun, but Indians were not prepared. While the Chinese weare heavily prepared, Indians were confident that war would not be triggered and made little preparations. On 20 October 1962, the Chinese army launched two attacks, 1000 kilometres apart. In the western theatre, the Chinese army sought to expel Indian forces from the Chip Chap valley in Aksai Chin while in the eastern theatre, the Chinese army sought to capture both banks of the Namka Chu river. Some skirmishes also took place at the Nathula Pass, in Sikkim. Gurkha rifles travelling north were targeted by Chinese artillery fire. After four days of fierce fighting, the three regiments of Chinese troops succeeded in securing a substantial portion of the disputed territory.
Over the following days, In the eastern theatre, there were clashes between Indian and Chinese patrols at Walong as the Chinese rushed in reinforcements. On 25 October, the Chinese made a probe, which was met with resistance from the 4th Sikhs. As some Chinese soldiers began to close in, Sepoy Kewal Singh charged them with his bayonet and killed a few of them in hand-to-hand combat, but he himself was killed. The following day, a patrol from the 4th Sikhs was encircled, and after being unable to break the encirclement, an Indian unit sneaked in and attacked the Chinese flank, allowing the Sikhs to break free.
In the western theatre, after stopping the Chinese advancement for days, the Indian Western Command withdrew many of the isolated outposts to the south-east. Daulet Beg Oldi was also evacuated, but it was south of the Chinese claim line and was not approached by Chinese forces. Indian troops were withdrawn in order to consolidate and regroup in the event that China probed south of their claim line.
Zhou sent Nehru a letter, proposing a negotiated settlement of the boundary that both sides disengage and withdraw twenty kilometres from present lines of actual control. Nehru's replied expressing interest in the restoration of peace and friendly relations and suggested a return to the "boundary prior to 8 September 1962". He was categorically concerned about a mutual twenty kilometre withdrawal after "40 or 60 kilometres of blatant military aggression". He wanted the creation of a larger immediate buffer zone and thus resist the possibility of a repeat offensive. Zhou's reply repeated his offer.
Facing Chinese forces maintaining themselves on Indian soil and trying to avoid political pressure, the Indian parliament announced a national emergency and passed a resolution which stated their intent to "drive out the aggressors from the sacred soil of India". The United States and the United Kingdom supported India's response. With the backing of other great powers, a letter by Nehru to Zhou once again rejected his proposal.
After Zhou received Nehru's letter rejecting his proposal, the fighting resumed on the eastern theatre on 14 November when Indians attacked on Walong, claimed by China, launched from the defensive position of Se La and inflicting heavy casualties on the Chinese. The Chinese resumed military activity on Aksai Chin and NEFA hours after the Walong battle.
Chinese troops probed further in the eastern theatre. While on the western theatre, Chinese forces launched a heavy infantry attack on 18 November near Chushul. Indians suffered heavy casualties, with dead Indian troops' bodies being found in the ice, frozen with weapons in hand. Chinese forces also suffered heavy casualties, especially at Rezang La. This signalled the end of the war in Aksai Chin as China had reached their claim line– many Indian troops were ordered to withdraw from the area. China claimed that the Indian troops wanted to fight on until the bitter end. However, the war ended with their withdrawal, so as to limit the amount of casualties.
On the eastern theatre, the Chinese penetrated close to the outskirts of Tezpur, Assam, a major frontier town nearly fifty kilometres from the Assam-North-East Frontier Agency border. The civilians in Tezpur were evacuated to the south of the Brahmaputra River.