Final answer:
Imperialism was a key cause of World War I as it led to increased competition and conflicts among European powers, who were driven by nationalist and militaristic incentives to maintain and expand their empires, often through violence and exploitation.
Step-by-step explanation:
Imperialism was one of the main causes of World War I for several reasons. It fueled competitive empire-building among European powers, leading to territorial disputes and escalated tensions. As Britain and other countries sought to maintain and expand their empires, imperialism often involved significant violence against colonized peoples, which was a reflection of the powers' willingness to use force to achieve their goals. This violence is exemplified by the atrocities committed in the Congo and the genocide of the Herero and Nama peoples. Furthermore, during the period of modern imperialism, Europe's control over vast territories was facilitated by a 'perfect storm' of industrialization, technological superiority, and ideologies such as Social Darwinism, which justified conquest.
The aggressive nationalism fostered by imperialism and the corresponding military build-ups, particularly the anglo-german naval arms race, contributed to an environment ripe for conflict. By 1914, long-building imperialistic and nationalist tensions enveloped Europe in war. Thus, imperialism did not just stimulate nationalist sentiment and militarization but also featured in the intricate web of alliances that bound countries to defend each other, contributing to the widespread nature of the conflict.