It varies by country, but let me discuss the case of India, since I know more about it.

Europeans arrived in India in 1498, with the discovery of the sea route by Vasco da Gama. In 1614, the Mughal Emperor Jehangir allowed the British to set up permanent trading posts. But it wasn't until 1757 that they gained their first inch of territory.

In the late 17th century, the Mughal Empire fought a series of wars with the Maratha confederacy, which weakened and bankrupted both sides, destroying any central power capable of exerting influence over a large part of the subcontinent. In Bengal, the local Nawabs had been vassals of the Mughals, but after the destruction of Mughal power by the Marathas, they became semi-independent, and a struggle for succession followed. The British aided one faction against the other in the hope of gaining favorable trade deals with the winner, but a traitor in the Bengal court named Mir Jafar gave them opportunity to take over territory for themselves. This was the beginning of Company rule in India in 1757, from the province of Bengal.

Over time, the British repeated the success of this ploy. After the long rule of the Mughals, there were no longer any well-established political entities with universally accepted rulers, so local strongmen fought each other bitterly over the pieces of the former empire. This was opportunity for the British to make deals with one ruler against his rival, join forces to win the battle and divide the spoils. There was not much resistance internally, because to the Indians, the British were an unknown entity with no history and no prior cause for hatred, whereas the local enemy was well-known and much hated. In this way, the Company slowly expanded its rule until it covered more than a third of the country by 1857, the year Company rule ended after the Indian mutiny.

After the British Crown took over from the East India Company, it actively maintained the Company policy of “divide and rule”. Because of the centuries old history of Muslim invasions, there were bitter feelings between Hindus and Muslims, which the British reinforced and exploited to keep them divided. Hindus had the caste system, which the British also used – promoting certain casts selectively, giving them privilege over others. This made other casts resentful and gave the promoted caste perks worth defending, leading to increasing inter-caste rivalries. They were also fond of using exploiting ethnic conflicts, bringing in Sikhs regiments to shoot at Hindus, bringing in Gorkhas to shoot Sikhs, Pastuns to shoot at the Gangetic populations. In this way, the Indian population remained divided, hating each other more than they hated British rule. The British did this in many other countries too, for example the Hutu and Tutsi in Rwanda.

But it was a precarious rule. At their peak, there were about 40,000 British in India, compared to a native population of around 300 million. They were far too few to rule such a populous country without the active help and participation of Indians, who formed the bulk of the police and military. As the British set up a centralized administration, they needed large numbers of bureaucrats to oversee it, and most of these also came from the Indian population. It was a system with British at the top, making the important decisions, but with the Indian population doing the work of administering and enforcing those decisions.

There were a number of factors that led to decolonization, and I’ll list a few of them:

Education

The British needed clerks capable of administering a land of 300 million people. The British had maintained a strong policy of discouraging local education during Company rule, as they rightly suspected it would foment rebellion. But under Crown Rule, they were forced to change this policy. There was a difference in philosophy between Company Rule and Crown Rule. Under the EIC, the British thought of themselves more as raiders, there to extract as much wealth in as short a time as possible. Administration was not a concern; it was more or less a continuous military campaign to bring as many princely states within Company Rule as they could. But the Crown looked upon India as a colony, an integral part of the British Empire, which would always be British, where British people would spend their lives. They were interested in having a stable and cooperative society which would accept this arrangement over the long term.

So they needed to create a class of clerks and administrators, which they did by setting up schools. These were minimalistic schools, offering only enough education to be a clerk, and no more. But once people start getting an education, it’s hard to stop how far it goes. Many Indians who got the taste for it left India to pursue further education abroad, often in England.

This changed the outlook of Indian people. As I said, the British had been an unknown entity, foreigners who suddenly appeared, fighting with this side or that against local rulers. There was little consistency in their method that Indians could see, sometimes they supported Muslim rulers, sometimes Hindu rulers. They were hard to categorize, hard to like or dislike based on which faction they favored. But after Indians started getting a western education, and especially after they began to travel abroad in large numbers, the view changed. Now the British came to be seen as colonizers, who hadn’t just created a colony in India, they were doing it all over Asia and Africa, and the Americas. Along with French, Dutch, Spanish, Portuguese. There was a commonality to their methods – they were European people who would go and subjugate brown or black people in their own lands and keep the locals enslaved.

This led to two things – a hatred of European colonialists, and a sense of unity with their own kind, with whom they shared a common fate. Old differences started diminishing in the face of a more unified opposition to foreign rule. This is the real start of the Indian freedom movement, in the early years of the 20th century.

Industrialization

As industrialization came to India, it became increasingly easier for Indians to match the British in the force of arms. Besides the millions of weapons issued to Indian police or military, it was increasingly easier to set up a cottage industry to manufacture them. Or the other things that made the British powerful, like their factories that produced manufactured goods. You could just as easily set up factories in India and create your own wealth.

The British tried their best to discourage such things. Indian factories were nationalized in the hundreds or thousands, it became harder for Indians to start factories or to import technology from Europe. But it was one more war for the British to fight, and they were losing it in the decades prior to independence, with laws being slowly liberalized to permit more Indian factory ownership, eroding the economic advantage the Brits enjoyed.

Internal Violence

As the independence movement gained strength, the British had to resort to desperate measures to keep things under control. There were several notable cases such as Jalianwala Bagh, where the British opened fire on a crowd of unarmed men, women and children, mowing them down with machine guns because they feared that these people gathered for a religious festival might secretly be planning insurrection. They also toughened laws which made it easier to throw Indians in jail without trial or recourse.

Such things made Indians angrier and provoked further violence. The independence movement, which had largely been peaceful so far, gained factions which were determined to overthrow British rule by force.

Wars

Wars may not have been responsible for the freedom movement, but they certainly accelerated it. In the case of India, it was not so much that Britain was weakened by the wars as India was strengthened. Britain is a small country, and in order to fight the Axis, they needed soldiers, which they conscripted from their colonies in huge numbers. Before WW2, the Indian army consisted of 200,000 men. By the end of the WW2, it numbered over 2.5 million. These were not play-soldiers, they were battle hardened veterans of numerous battles with the Japanese in southeast Asia, and were familiar with modern warfare.

So what started as an asset for the British to help win the war, ended up as a liability. There was no possible way that a tiny number of British officers could control an army of 2.5 million soldiers if the soldiers ever united against them. And this is in fact what happened.

During the war, a small fraction of Indians had sided with the Japanese, on the grounds that “the enemy of my enemy is my friend”. Subhash Chandra Bose led this faction and offered help to the Germans and Japanese to help defeat Britain. It was a very small army that mostly fought in Africa, and didn’t influence the course of the war in any discernible way. But to the British they were traitors and had to be punished. So after the war, some of their officers were tried, and a few hanged.

This angered the Indians, who didn’t see them as traitors. To them they were just freedom fighters who opposed the British, perhaps in a misguided way, but not deserving death. The resentment spread, and in succession, the Indian Air Force, Army and Navy rebelled. The British quickly negotiated a peace, but the unthinkable had happened. The Indian military could no longer be trusted to carry out instructions from their British officers, and they were too large a force to be ignored. This was reported in British Parliament, and from that point on, it was clear that they would have to leave. In fact, after that they couldn’t get out fast enough. Withdrawal was actually accelerated, and the British left in such a hurry in 1947 that many promises they had made before independence had to be broken.

[..... continued below]