Colonialism is the establishment, exploitation, maintenance, acquisition, and expansion of colony in one territory by a political power from another territory. It is a set of unequal relationships between the colonial power and the colony and often between the colonists and the indigenous population. The European colonial period was the era from the 16th century to the mid-20th century when several European powers (particularly, but not exclusively, Portugal, Spain, Britain, the Netherlands, Russia, and France) established colonies in Asia, Africa, and the Americas. At first the countries followed mercantilist policies designed to strengthen the home economy at the expense of rivals, so the colonies were usually allowed to trade only with the mother country. By the mid-19th century, however, the powerful British Empire gave up mercantilism and trade restrictions and introduced the principle of free trade, with few restrictions or tariffs.