When Mountbatten left England for India he had received instructions to work towards a unitary government based on the Cabinet Mission's plan, but to prepare for alternative steps, if by October 1st, there was no prospect of such a Government. Now that the Mission's plan had fallen by the wayside, a plan for a truncated Pakistan was adopted, a larger Pakistan no option as the Muslim League was unable to take it by force and Congress would not allow it by agreement.
Mountbatten got to work immediately, aided by Wavell's well balanced and experienced staff in designing a partition for Pakistan and India which should be agreeable to both parties. The main problems being the party leaders, along with Gandhi in the background of Congress politics. Gandhi was intractable over a partition, particularly one effected under British aegis, Nehru was excitable and prone to rash public statements while Jinnah was adept at saying no to every proposition put to him, was vain, obstinate and intractable.
Mountbatten went to great lengths to humor Gandhi, whom he believed was mainly responsible for the wrecking of the previous plans of the Viceroy and British Government to create an Independent India. Mountbatten was skilled in removing irritants and keeping the plan moving smoothly as he handled the side issues in his first weeks of office. One of these was the issue of compensation for officials who would, in theory, be made redundant when Independence occurred. The matter eventually being solved by offering terms to British officials, who would return to Britain anyway, but the Indians were not to be offered compensation as they were likely to remain in post and probably gain from Independence as their British colleagues left.
Mounbatten was anxious that partition should be shown to reflect the will of the Indians themselves, each province deciding if it wanted to go along with the new constitution as framed by the assembly, a new Constituent Assembly for Pakistan or their own Constitution. Methods of gaining a feeling of the popular opinion varied within the provinces. If a decision was taken to partition the Punjab, a referendum would be taken to decide where the North West Frontier Province would end up. In Bengal, representatives would decide whether the district of Sylhet would be part of Assam or would be amalgamated into the Muslim part of Bengal. In Sind, the choice between the three options would be determined by the Provincial Legislature. The plan was shown to the principal leaders of India, Jinnah objected to the partition of Bengal and the Punjab, but indicated he would accept rather than go without Pakistan altogether. The final draft of the plan was returned from London, reworded, and rejected by Nehru. This was then reworked by V.P. Menon and sent to England while the Indian leaders considered it and finally accepted the plan on 3rd June. This version eliminated the option of Provinces to remain independent in order to placate Nehru. On 18th July, the Indian Independence Bill was enacted by the British Government, and voting proceeded smoothly in the Bengal and Punjab Provinces and resulted in the decision that both these Provinces should be partitioned.
Partition occurred with both India and Pakistan becoming independent Dominions on August 15th and both receiving a portion of the Indian Army, although the centralized administrative control remained under the command of Auchinleck until 1948, when each Dominion might be in a position to assume its own control. A Partition Council was set up to divide the assets and liabilities of India, composed of top ranking party leaders and province representatives. Each of the partitioned states, Punjab and Bengal, had a boundary commission to monitor the borders of Partition.
For the first months, to oversee the transfer of power, it was proposed that both Dominions have a common Governor-General. Nehru invited Mountbatten to be the first incumbent but Jinnah was opposed to the idea, he wanted to be Pakistan's first Governor-General, which was considered his own personal creation. Mountbatten accepted the post, for a limited transitional period, as Governor-General of India. The States were another matter, none part of India or Pakistan,they were encouraged to maintain cordial relations with the new Dominions as they had with British India.
During June and early July, Mountbatten had had to fight to hold the Interim Government together as Congress had wanted to evict the Muslims from power as soon as partition took place, their representatives holding portfolios for Pakistan only. However, once the Independence bill passed, both parties-with much difficulty-were convinced of the need for seperate provisional governments for both Dominions. The Pakistani provisional government moved to Karachi beginning on 1st August.
More rioting followed the voting for partition, Hindu and Muslims blaming each other for breakups of their country and communal violence occurred in Gurgaon, Lahore and Amritsar. Nehru laid the blame at the British authorities feet, for their failure to prevent the violence and this had the effect of discouraging most British officials from remaining in India service after Independence. Rather more continued in Pakistani service. In the Punjab more rioting and violence occurred between the Sikhs and Muslims, as the Sikhs sought to prevent their religious properties and lands from falling into Pakistan and tried to assassinate Jinnah. The border commission in the Punjab formed a force of 50,000 to control and police the Boundary between the two Dominions.
On August 14th, Mountbatten visited Karachi, and paid his respects to Jinnah and offered Pakistan his good wishes before leaving for Delhi to oversee the Inauguration of the Independent India at midnight the same day. In Punjab, however, August 15th, marked the beginning of two months of mass slaughter and a mass migration, unparalleled in world history. Existing conditions forced the migrations, although Sikhs and Hindus in Pakistan were assured they would be given religious freedom. In Lahore, part of Pakistan, the Sikhs and Hindus were instructed to leave a few days after partition and Muslims massacred every Hindu and Sikh on the trains bound for Amritsar and in relation, the Sikhs and Hindus killed the Muslims heading for Lahore by train. The Boundary force at this stage was less than 10,000 strong and totally inadequate against the Sikhs, nor was it reliable, having been infected by communal frenzy. Both Dominion Governments very shortly resigned themselves to protecting the migratory columns as best they could. The total casualties in the end were probably about 200,000 and over ten million people migrated. This migration took place between the middle of August and the middle of November, but from some outlying areas such as Sind and Baluchinstan the movement was more gradual and more peaceful.
India and Pakistan, Hindu and Muslim continue as rivals, sharing a mutual border over the area of what once was 'British India'.