New Imperialism was a period of territorial expansion by the countries of Europe, the U.S. and Japan which began around 1870 and lasted until the mid-twentieth century. During this time, these imperial nations colonised vast swathes of Asia, the Middle East and Africa.
There are a number of factors which explain this sudden expansion in the late nineteenth century. First of all, these imperial nations had all reaped the benefits of industrialisation: they were at the height of their economic, political and military power and wanted to demonstrate their wealth and prestige to the rest of the world. These nations also believed themselves to be culturally superior to other cultures, an idea we call 'ethnocentrism', and so expansion also provided them with an opportunity to transmit their norms, values and religious beliefs on other 'uncivilised' races.
The rise of New Imperialism had some important repercussions on these colonies. For a start, it destroyed many native customs and political and social institutions. The plundering of their natural resources also made these countries poorer as imperial nations grew richer.
Expansion also created rivalries between imperial nations which would have far-reaching consequences: conflict between France and Germany over Morocco and between France and Britain over the Sudan created the tensions which erupted in World War One.