In Chapter Twelve of A People's History, Zinn argues that while socialists and other radicals had opposed the Spanish-American War and the war in the Philippines, labor unions had been divided, with many pleased by the high wages brought by the war. Additionally, many African-Americans supported the war, which they saw as a chance to prove their patriotic bona fides.
In the following chapter, he claims that "war and jingoism might postpone, but could not fully suppress, the class anger that came from the realities of everyday life." Indeed, many radicals came to see the war as the ultimate expression of capitalist greed. Zinn points to evidence of rising class antagonism and political radicalism in the first decade of the twentieth century, trends which defied Progressive attempts to reform the capitalist system which Zinn considers utterly corrupt.
Zinn cites a number of developments in support of his thesis: the consolidation of capital into the hands of men like J.P. Morgan; terrible working conditions in the factories where new immigrant workforces toiled; violent strikes in urban centers; the emergence of the Industrial Workers of the World; and the assertion of rights by working class blacks. Above all, however, he describes the rise of what he describes as a broad-based socialist movement, one which had an "almost religious fervor" embodied by its most passionate leader, Eugene V. Debs, and others like Emma Goldman and Helen Keller.
The chapter concludes with an account of the violent "Ludlow Massacre" in Colorado, and, in the midst of all this turmoil and bloodshed, he closes by insinuating that the First World War (and the US military intervention in Mexico in 1914) might just have been motivated by a desire to reunify the country.
Source: Howard Zinn, A People's History of the United States (New York: Harper, 1995), 314-349.