Western European colonialism and colonization was the Western European policy or practice of acquiring full or partial political control over other societies and territories, founding a colony, occupying it with settlers, and exploiting it economically. For example, colonial policies, such as the type of rule implemented, the nature of investments, and identity of the colonizers, are cited as impacting postcolonial states. Examination of the state-building process, economic development, and cultural norms and mores shows the direct and indirect consequences of colonialism on the postcolonial states. It has been estimated that Britain and France traced almost 50% of the entire length of today's international boundaries as a result of British and French imperialism.

The era of European colonialism can be defined by two big waves of colonialism: the first wave began in the 15th century, during the Age of Discovery of some European powers vastly extending their reach around the globe by establishing colonies in the Americas, and Asia. The second wave began during the 19th century, centering around Africa, in what is called the Scramble for Africa. The dismantling of European empires following World War II saw the process of decolonization begin in earnest. In 1941, President Franklin D. Roosevelt and British Prime Minister Winston Churchill jointly released the Atlantic Charter, which broadly outlined the goals of the U.S. and British governments. One of the main clauses of the charter acknowledged the right of all people to choose their own government. The document became the foundation for the United Nations and all of its components were integrated into the UN Charter, giving the organization a mandate to pursue global decolonization.^{[citation needed]}

Historians generally distinguish two main varieties established by European colonials: the first is settler colonialism, where farms and towns were established by arrivals from Europe. Second, exploitation colonialism, purely extractive and exploitative colonies whose primary function was to develop economic exports. These frequently overlapped or existed on a spectrum.

Settler colonialism is a form of colonization where foreign citizens move into a region and create permanent or temporary settlements called colonies. The creation of settler colonies often resulted in the forced migration of indigenous peoples to less desirable territories. This practice is exemplified in the colonies established in what became the United States, New Zealand, Namibia, South Africa, Canada, Brazil, Uruguay, Chile, Argentina, Israel and Australia. Native populations frequently suffered population collapse due to contact with new diseases.

The resettlement of indigenous peoples frequently occurs along demographic lines, but the central stimulus for resettlement is access to desirable territory. Regions free of tropical disease with easy access to trade routes were favorable. When Europeans settled in these desirable territories, natives were forced out and regional power was seized by the colonialists. This type of colonial behavior led to the disruption of local customary practices and the transformation of socioeconomic systems. Ugandan academic Mahmood Mamdani cites "the destruction of communal autonomy, and the defeat and dispersal of tribal populations" as one primary factor in colonial oppression. As agricultural expansion continued through the territories, native populations were further displaced to clear fertile farmland.

Daron Acemoglu, James A. Robinson, and Simon Johnson theorize that Europeans were more likely to form settler colonies in areas where they would not face high mortality rates due to disease and other exogenous factors. Many settler colonies sought to establish European-like institutions and practices that granted certain personal freedoms and allowed settlers to become wealthy by engaging in trade. Thus, jury trials, freedom from arbitrary arrest, and electoral representation were implemented to allow settlers rights similar to those enjoyed in Europe, though these rights generally did not apply to the indigenous people.

Exploitation colonialism is a form of colonization where foreign armies conquer a country in order to control and capitalize on its natural resources and indigenous population. Acemoglu, Johnson, and Robinson argue, "institutions [established by colonials] did not introduce much protection for private property, nor did they provide checks and balances against government expropriation. In fact, the main purpose of the extractive state was to transfer as much of the resources of the colony to the colonizer, with the minimum amount of investment possible." Since these colonies were created with the intent to extract resources, colonial powers had no incentives to invest in institutions or infrastructure that did not support their immediate goals. Thus, Europeans established authoritarian regimes in these colonies, which had no limits on state power.

The policies and practices carried out by King Leopold II of Belgium as the absolute ruler of the Congo Free State in the Congo Basin are an extreme example of exploitation colonialism. E. D. Morel detailed the atrocities in multiple articles and books. Morel believed the Leopoldian system that eliminated traditional, commercial markets in favor of pure exploitation was the root cause of the injustice in the Congo.^{[page needed]} Under the "veil of philanthropic motive", King Leopold received the consent of multiple international governments (including the United States, Great Britain, and France) to assume trusteeship of the vast region in order to support the elimination of the slave trade. Leopold positioned himself as proprietor of an area totaling nearly one million square miles, which was home to nearly 20 million Africans.