Broadly defined, imperialism is the practice by which a group attempts to expand its influence beyond its borders. Often this takes the form of colonialism, the practice by which a group seeds outside territory with representative people and institutions, in an effort to expand control. Historically, imperialism and colonialism in the modern era are linked to the political and geographic expansion of European powers beginning in the fifteenth century and abating in the period of decolonization in the 1950s and '60s. Imperialism and colonialism as historical forces are also linked inextricably to the development of capitalist economies in Europe and abroad.
Imperialism was an important concept in Du Bois's world view. His seminal essay “The African Roots of the War” ran in The Atlantic in May 1915. In it, Du Bois located the basic motivating impulses of the Great War in Europeans' previous contests for and conquest of the African continent. “The African Roots of the War” also demonstrated Du Bois's belief that race and racism were important components of European imperialism, not merely byproducts.
One of the chief crimes of imperialism, according to Du Bois, was its tendency to exploit the labor of indigenous peoples—most often people of color—especially for profit in capitalist economic systems. As in many other critiques of European imperialism, Du Bois pointed to profit as a key impulse animating Europeans in the colonial era.
Du Bois also perceived imperialism as an animating force in the Cold War, the international rivalry between the blocs of nations led by Soviet Union and the United States following World War II. To Du Bois, the United States' foreign policies in the Cold War era were attempts to revivify the British imperialism of the preceding century. 
Du Bois's theorizing about the relationship between imperialism and capitalism had much in common with that of Vladimir Lenin, though Du Bois made surprisingly few references to the Russian leader in his letters and published works. Notably, Du Bois's “The African Roots of the War” preceded Lenin's famous Imperialism, the Highest Stage of Capitalism by two years. Both Du Bois and Lenin drew on the idea of Karl Marx, the German theorist credited with establishing the basic precepts of socialist political science.
 Du Bois, W.E.B. Peace is Dangerous. New York: National Guardian, 1951.