The easy answer is the turmoil of two World Wars and a massive Economic Depression, however while that is true they are only the impetus for three more important factors. These three factors are: 1. ) the economic concerns of two massive wars and a Great Depression; 2. ) a series of home-grown Nationalism movements; and 3. ) the political interests of two World Powers (US vs. USSR). These factors not only encouraged economic freedom and cooperative action against the colonizers, but made continued colonization an unattractive option to most powers.
Nothing is as unattractive as continued expense for little gain. Prior to the First World War most Empires enjoyed a significant economic boost from their colonies. During the Great War the colonies acted as reservoirs of both resources (ammunition, rubber, funds) as well as natives to press into military service. This changed as several issues came into play. The first problem was the return of those native soldiers and their expectations of colonial independence for service given. The disappointment of those expectations served as a strong backing to growing nationalism.
The second problem was massive costs racked up both in terms of human lives lost and debts raised. As an example Britain paid out an estimated 30 percent of the total cost incurred by the Allied Forces. That would be over 38 billion USD without adjusting for inflation between 1918 and 2013. France did not fare much better with their estimated share of 22 percent of costs. Besides these direct costs and the massive loss of life, both these powers also had loans to repay to the United States.
However, both countries were big enough players that they could afford to not only cling to their stakes, but to continue colonizing. On the other hand Germanys burden of debt plus staggering war reparation payments combined with a sluggish economy to effectively end their imperialist aims. The third and arguably most important issue was the shifting of economical focus in the colonies. Prior to the Great War and the Great Depression that followed, many colonies were focused on growing cash crops.
However, agricultural products saw a far greater plummet in value at a much quicker pace than industrial goods. Unfortunately for the colonies domestic concerns trumped their well-being and they were left to suffer along as best they could. Lacking export income the colonies were forced to try different avenues some returned to subsistence farming, some diversified, and a few turned to industrialization. On top of that many European-backed plantations found themselves vulnerable allowing for a reduction of minority dominance in both the economy as well as the government.
The end result were colonies that did not economically fit into neat categories, required a more direct approach, and had a growing class of bourgeoisie elite. The bourgeoisie elite owed their existence to several factors: a changing economy, a bureaucratic government created by the need for a more direct approach, and a sort of forced segregation. The most important point about the bourgeoisie is that they tended to better educate their children. These children were often schooled in Western establishments exposing them to the idea of self-determination as well as liberties not available under colonial government.
This leads to the formation of an intelligentsia who would not only found, but lead many of the nationalist movements. The general appearance and strength of these movements grew in the period between War World I and II as promises of self-rule were tossed to the wayside in spite of President Woodrow Wilsons championing of the idea of self-determination. Wilsons 14 point speech appeared to indicate a strong position of decolonization on the part of the United States.
At one point it assured former colonies of the Ottoman Empire that they would quote: (be) assured¦ n absolutely unmolested opportunity of autonomous development. At another point he said this of his proposed League of Nations, that it would give mutual guarantees of political independence and territorial integrity to great and small states alike. In the end the US didnt join the League of Nations which would devolve into a front for colonialism. However, the ideals espoused were taken up by the nationalist groups as a way of adding legitimacy to themselves. In turn these organizations were able to overcome ethnic differences in order to create legitimate nations a well as coordinate large scale displays of resistance (peaceful or otherwise).
Of course, most fledgling nation states cant begin to hope for independence without some degree of foreign assistance whether political or economic. An easy example of this is the aid the American colonies received from France during the Revolutionary war. In counter point is the aid given Haiti from Britain in their war against France. As can be seen from those two examples, the aid rendered a colony is often less a factor of altruism as it is an act of sabotage to the interests of the colonizing country.
However, another reason for helping a colony gain independence is to effectively spread your ideology and/or political influence. It should therefore come as no surprise that following the end of War II, when the US and the USSR arose as Superpowers they set about fighting imperialism. Historians Alan Sked and Chris Cook have made the point that, the United States had the luxury of keeping its material interests in line with its ideology. In other words the US was able to continue under the banner of self-determination whilst curtailing the recruiting efforts of the USSR.
On the other hand, the USSR could claim to be fighting on behalf of the common man by preventing the instigation of a class-based society as a result of capitalist imperialism. If the process of rendering assistance also managed to gain the Soviet Union more satellite states while eroding the powerbases of potential enemies that was just a bit of a bonus. Either way battles for independence by these colonies ended up becoming battlegrounds for the interests of these superpowers during the Cold War. That said, these two were also capable of working together as shown by the pressure they exerted on Britain to decolonize.
The US went as far as making such a condition of the Lend-Lease Act of 1941, knowing that Britain desperately needed the supplies they would get in exchange. Britain is perhaps the best example of all the points made thus far. At its zenith the British Empire consisted of ? of the Earth. However, by 1945 it lacked the resources to retain its possessions. Back to back wars had left the country with staggering debt, and the damage left by the 2nd war resulted in a weakened infrastructure for example Britains once great Navy was in tatters.
This not only left Britain without unable to police its colonies, but meant that the previous reliance on civil servants could not be sustained. There was nothing in the coffers to pay them with. So right there you have economic issues. Meanwhile Mahatma Gandhi was using the nationalistic feelings of his country to stage a massive wave of civil disobedience. With each success India became a rallying point for other British colonies. So there is the nationalistic trend. Finally, as previously mentioned the US and the USSR had Britain over a barrel.
With little real choice Britain had to relinquish their various holdings. And the other colonial powers would one by one follow suit. Some of them would engage in violent, protracted fighting. Others would try compromise and political maneuvering. Yet others like Japan would simply cede all their colonies as a term of surrender. However, they came about decolonizing the factors involved were generally based on economic concerns, strong native nationalist movements, and depending on the time period the interests of either the US or the USSR (if not both).
Not that this was much of an issue overall. In general decolonization allowed for the benefits of colonies (cheap labor and goods) without the attendant concerns of maintaining them. John Kenneth Galbraith perhaps put it best in the book, In A Journey Through Economic Time, when he said: The end of the colonial era is celebrated in the history books as a triumph of national aspiration in the former colonies and of benign good sense on the part of the colonial powers. Lurking beneath, as so often happens, was a strong current of economic interest ” or in this case, disinterest.