Many nations around the world were colonies of mainly European countries until recently, when they gained their independence by the process of decolonization which ultimately hands over and grants sovereignty to the former colony. The transitional period may be marked by turmoil and violence, warfare or peace, and often the newly independent countries face difficult post-colonial adjustment problems and may face "neocolonialism" or being interfered with by their former colonial power.
Independence movements reach maturity often after wars between empires and secondarily after independence efforts in neighbouring lands (via a 'demonstration effect.'). For example, independence for the United States and India had far-reaching observation effects, emboldening people elsewhere to act once they could point to real examples of political change. In the 20th century, defenders of overseas empires were placed ideologically and morally on the defensive. As each new state entered the United Nations, the organization became a forum for anti-colonial lobbying. It hastened the Dutch departure from Indonesia, for example.
Virtually all former colonies are now independent countries. The UN Trusteeship Council has been central in overseeing the transformation of colonies into countries.
21st-century decolonisation is concerned with the goal of attaining what the Kenyan writer Ngugi wa Thiong’o in 1986 called ‘decolonising the mind’: first, becoming aware of and rejecting the coloniser’s system of knowledge that still lingers today; then, replacing it with more balanced, diverse, complex and locally relevant understandings of human societies and the relationships between them; and, finally, changing practices as a result. It’s a long and tortuous process, much more so than replacing one flag with another, or swapping one national anthem for another.