One policy that European countries have used in the past to build an empire is colonialism. This policy involves the acquisition and control of territories outside of the colonizing country's borders through exploration, conquest, and settlement. European powers such as Spain, France, and Great Britain used colonialism to establish overseas empires in the Americas, Africa, and Asia, where they exploited the resources and labor of the colonized peoples for their own economic gain.
Colonialism also led to the spread of European culture, language, and institutions to the colonies, often at the expense of the indigenous cultures. European colonial powers also established trade networks and infrastructure to extract resources, which in turn boosted their economic growth.
Another policy could be imperialism, which refers to the practice of extending a country's political and economic influence over other territories and peoples through means such as military conquest, diplomacy, and economic control. This can be distinguished from colonialism in that imperialism can also occur through non-settlement means.
It's important to note that these policies were carried out without respect for the rights and well-being of the colonized and indigenous people, and resulted in the exploitation, displacement, and oppression of millions of people. The consequences of this historical legacy are still felt today.