This is a question which can only be fully answered over the pages of several books since the formation of the India Union spans nearly two centuries and the the geopolitics of the subcontinent is equally broad and complex. What ill attempt to do here is provide a summary that goes through various political, social and strategic processes that led to the contemporary situation in the subcontinent. Since I specialise in the history of the Indian Union the summary I give is primarily a history of the same entity. I wont be commenting much on Pakistan, Bangladesh and Srilanka for the same reasons.

At the end of colonial rule, British India was a vast territory that stretched from Afghanistan to Burma. Burma had been made a separate crown colony in 1916. The modern political geography of the current nation states of the Indian subcontinent derive from the British India of the early 20th century. Now British India was culmination of a variety of forms of political control. There were the Presidencies in Bombay, Calcutta, Delhi and Madras and the territories under these presidencies. This was under the direct control of the monarch of England from 1858. Interspersed between this territory were the Princely States which were ruled by local monarchs but were controlled by British Resident or Political Officer. At the time of Indian Independence these Princely States numbered around 556. It is within this political system that Indian nationalism began to emerge and grow. The main political entity in the Indian national movement was the Indian National Congress founded in 1885 but it is not the only anti-colonial entity that was active in creating the Indian identity. There were a variety of regional interests that created their own discourse of nationalism, nativity and anti-colonialism . The transportation, communication and political systems of British colonialism had made it easier for the subjects of British India to interact and share ideas to form a coherent identity. The modern Indian identity is not formed overnight but it took nearly a 100 years worth of poetry, literature and political activism to create the consolidated idea. It is also not a monolithic identity there are several sub-nationalisms that contribute to the larger idea. Its better to understand the Indian identity as aggregation rather than a singularity.

Recommended Reading : Sumit Sarkar- Modern India Shekar Bandopahyay- From Plassey to Partition

In 1947 India gained Independence from colonial rule. As political negotiations to keep India integrated with the same political geography as British India fell apart and the country split into two. Pakistan(which in 1947 was west-Pakistan and east-Pakistan which is present day Bangladesh). Minus this territory most of British India began to form the Indian Union. Now the present day political map of the Indian Union is not what the country looked like in 1947. In fact constant shifts occurred in the territory of India till 1976 (2016 if you count enclaves). In fact contrary to mainstream Indian historiography the Indian Union is the product of a series of negotiations conflicts and insurgencies. Initially several princely states objected to joining the Union. The state of travancore wanted to remain independent but a popular left wing uprising overthrew the monarchy and voted to join the Union. The state of Hyderabad was annexed by the Indian army in 1948, essentially because the state would have been a country within a country with strong ties to Pakistan. The princely state of Junnagad became a site of conflict as to if it would join Pakistan (because of the interests of it muslim monarch) or join India in the interest of its hindu majority(the latter was what happened). The reverse was true of Kashmir which had a Hindu monarch who after a speedy referendum ceded to India. Stating the muslim majority argument Pakistan invaded what is today azad kashmir/Pakistan occupied kashmir. This issue as you may know is still unsettled. There were other political negotiations that the Indian Union had to undergo to tip the balance in the French controlled territories of pondicherry, yanam and mahe. These territories would referendum out of colonial rule and accede into the Union. The Portuguese colony of Goa would also be absorbed into the union in 1961 through strategic military move by the Indian army. The latest change to the map of the Indian union happened in 1976 when the princely state of sikkim was made an Indian state. Sikkim had been an Independent kingdom during British rule. The Indian union had turned sikkim into a protectorate after the Indo-china war of 1962 taking over military and diplomatic affairs of the kingdom. In 1975 a controversial referendum and social strife against the monarchy in sikkim saw it absorption into the Union. Recommended reading- vp menon- The Story of the Integration of the Indian States Smash and Grab- Sunanda Ray Andrew Duff-Requiem for a Himalayan Kingdom

As you can see from these examples there is a complexity of factors that created the present day Republic of the Indian Union.

How did India avoid escalation of conflicts into civil wars?

There is no real answer for this. In fact the Indian union has always contained problems that in other nations would have definitely escalated into civil wars (or in India multiple civil wars). But the fact remains that the republic has always been strong and none of India’s internal conflicts have ever caused a breakdown of the entire system. This could be primarily because of how the Indian Union is designed. Unlike a standard nation state India is not a monolithic governmental entity. One allegory(not comprehensive) is to say India is like the European Union but with a state structure similar to the United States. Like the EU India is made up of several nationalities but like the US its it has a centralised government below which function very strong state governments. These state governments are small nations in themselves but military and economic policy is with the central government. This form of power sharing is significant reason for the sustenance of the Indian Union. Though demographic differences do favour certain cultures and linguistic groups in the union the states still hold significant autonomy. In fact except for a few cases the solution in the Union for conflict is often to co-opt opposing parties and give more self governance. By doing this the Union never escalates a conflict beyond a certain point.

At the same time this system is not without its fractures. A significant number of secessionist conflicts dot the history of contemporary India. Everyone knows about kashmir so I wont delve further into that. Apart from this India has had secessionist movements for several regions. The conflicts in north-eastern india are significant in this regard. Having been mostly outside direct colonial control and not fully linked to the politics of Indian nationalism most states in north-eastern India remained socially, culturally and linguistically different from the mainland. The political entity that was once British Assam further turned into 6 different states in the Union. All of these states have their own secessionist movements. Naga nationalism which still survives today in a few small insurgent groups hoped to create a separate nation ‘Kuknalim’, joining different naga tribes in Myanmar and the Indian states of Nagaland, Manipur, and Arunachal Pradesh. The Garo Liberation army pushed for separate state in Meghalaya and the Mizo National Liberation Front was formed to create a separate Mizo nation. Besides this there are several secessionist movements or movements that seek to form new Indian states like the United Liberation Force of Ahom, Boro Liberation Tiger Force. In the mainland there is a long history of maoist/marxist militias that span central, eastern and southern India. These groups consider the Indian state a semi-colonial, semi-feudal, capitalist entity and have been in conflict with the central government for decades.

All this has led to the Central government developing a series of military divisions that are deployed for a variety of conflicts. Unlike most countries Indian armed forces are of two type. The Indian military (Army/Navy/Airforce) which is controlled by the Defence Ministry and the Reserve Police controlled by the Home ministry. The Reserve Police play a significant role in the regulation of domestic conflict. They are essentially the counter balance. Within the reserve police there are divisions for border patrol, the Border Security Force for the Bangladesh/Pakistan borders, Indo-Tibetan Border Police, Sahastra Seema Bal(nepal-bhutan border), Assam rifles(Myanmar border), and the Coast Guard for the seas. For engaging in domestic often secessionist conflict the central government deploys the Central Reserve Police Force and the Rapid Action Force is a division sent for riot and crowd control. There are also division like the Central Industrial Security Force and the Railway Protection Force which is for infrastructure security. Apart from the co-optation and political/diplomatic negotiation the reserve police are the other side of how India ensures internal integrity. Most internal conflicts in India are fought by reserve police not the army. Recommend Reading – Steven Wilkinson- Army and Nation