The unhappy experience of the Democratic Party during the Civil War was surely not the only cause of the reluctance of major parties to embrace dissent in subsequent wars. The discipline and cohesion that modern industrial societies impose upon their populations, in contrast to less-centralized and more loosely organized agricultural societies, had already helped maintain the united front of the North during the Civil War, and that discipline of industrialism had grown immensely stronger by the time of the great world wars of the twentieth century. In World War I, the unity of the populace of every major power in support of nationalist war and patient endurance of the populace through prolonged war confounded the expectations of numerous prophets who had forecast that modern wars would be so costly that they could last only a few months. By World War II the public discipline of the great powers had become even more impressive. Democratic America certainly was no exception to this pattern; if anything, it was the democracies that displayed the greatest social cohesion. In both world wars, dissent in the United States was confined to minuscule fringe groups, mainly of socialists, radical leftists, and pacifists, and conscription focused far more attention upon the conscientious objector than it had done in the Civil War. This change occurred not only because the government and the public demonstrated increasing sensitivity to the demands of the pacifist conscience, but also because the conscientious objector was much more nearly alone as a dissenter than he had been in 1861–1865.
Before the entrance of the United States into World War I on 6 April 1917, it is true, there had been significant dissent over the course in foreign policy that proved to be leading to war, most conspicuous among the protesters being the Progressive elements of President Woodrow Wilson's own governing coalition. Although after the war the intractable realities of international politics were to cause among this coalition a speedy disillusionment with support for the war, nevertheless from Wilson's war message onward throughout the war itself, all but a small fraction of this group were enthralled by the president's promise that the fight was for a reformation of the whole world, and they joined ranks behind the war effort. Not surprisingly, however, the considerable dissent that had surrounded Wilson's foreign policy before the war contributed to an expectation that there would be more dissent during the war than actually materialized. This expectation in turn contributed to passage of stringent espionage and sedition acts seeking among other things to suppress any utterances that might discourage recruiting or the united prosecution of the war. These acts were enforced not only by an enlarged body of federal investigative agents but also by the federally sponsored American Protective League of private citizens. Abetted thus by what amounted to vigilantes, attacks upon civil liberties and particularly upon free speech became absurdly disproportionate to a mere trickle of dissent. No cases involving wartime suppression of dissent reached the Supreme Court until after the war. Then Justice Oliver Wendell Holmes's characterization of Abrams v. United States (1919) as involving merely a silly leaflet by an unknown man came close to what is likely to be the historian's view of all the targets of the espionage laws; but in Abrams, Holmes spoke for the minority of the court, and he himself had seen "a clear and present danger" in the earlier and not much different Schenck case (1919).
A greater sophistication and tolerance marked the government's attitude toward dissent in World War II—always excepting the relocation of more than 100,000 Japanese from the Pacific Coast. Tolerance could well be afforded. Although American entry into World War II was also preceded by much debate over the nation's course in foreign policy, the Japanese attack on Pearl Harbor assured that from the beginning of direct American participation there would be still less dissent than there had been in 1917–1918. Although such a leading Republican spokesman as Senator Robert A. Taft of Ohio suspected Franklin D. Roosevelt of exploiting the war emergency to fix the changes of the New Deal permanently upon American institutions, he and most other Republicans permitted themselves only the most cautious criticism of the conduct of the war, which they carefully distinguished from criticism of the war itself. "Every problem," said Taft, "must be approached in a different spirit from that existing in time of peace, and Congress cannot assume to run the war"—a far cry from the attitudes even of members of Congress of Lincoln's own party during the Civil War, let alone the opposition. In World War II as in World War I, a certain amount of trouble did develop between the government and the labor movement, over labor's threats to strike to ensure itself a due share of war prosperity; but this friction can hardly be said to have involved dissent over the war. In World War II, national unity survived even though the American participation lasted nearly four years and was by American standards costly in casualties, unlike the American participation in World War I. The most evident explanation for this national unity was the nature of the enemy and of the circumstances with which the war began. But the historian seeking the sources of unity should also keep in mind that once the initial defeats were overcome, it would be hard to find a long war marked by so consistent a record of military success as favored America and surely helped sustain American morale in World War II.