The result of World War I was the dismembering of three empires that had existed for centuries in Europe, the Middle East, and North Africa: the Austrian, Ottoman, and Russian Empires. All of these had been multiethnic entities in which a single group ruled over other nationalities, despite (in some cases) varying degrees of autonomy some groups did have within those empires.
Though the effects of this disassembling of imperial states were partly reversed (for instance: the Soviet Union recaptured much of the non-Russian territory it had lost, and Turkey regained the parts of Asia Minor in which the Greeks and the Armenians had set up states) in general much of the change was irreversible. In Europe, nationalities such as the Czechs, Slovaks and Hungarians, which had been part of the Austrian Empire for centuries, were now independent. The Arab territories were now severed from the Ottoman Empire and were made into new countries, though still for the time under European "mandate."
It was reasonable that under these conditions, people elsewhere in Asia and in Africa who had been colonized would expect that they, too, should and would be granted independence. Self-determination of nations had been one of Woodrow Wilson's Fourteen Points and was implicit in the treaties that concluded World War I. At the same time, the concept of nationalism had been a driving force in European and world culture for at least a century. Why, most people in countries still under the colonial system thought, should some nations be given the right of self-determination and independence but not others?
Unfortunately it would take another 25 years and another devastating world war for the Europeans to begin the decolonization of India and other countries. Yet it wasn't a forgone conclusion even then that this would necessarily take place. In 1945 a Labour Government was elected in Britain which was in favor of granting Indian independence. Had Churchill and the Conservatives remained in power, decolonization would almost surely not have occurred as early as 1947. And it took a further ten to fifteen years before independence was granted the colonies in Africa.
The French held on as long as possible to Indochina (Vietnam) until being driven out in 1954, and when DeGaulle finally made the decision to pull out of Algeria in the early 1960's, the right-wing opposition in France were so incensed that they made multiple attempts to assassinate him. So, the ending of the European imperialist system was a protracted and difficult process.