Somebody had to carry the blame. But who?

That question would dominate the political debate of the interwar years. The response offered by many postwar critics of President Woodrow Wilson’s war leadership would slow the U.S. response to the rise of Adolf Hitler—and cast an influence over foreign-policy debates into our own time. The criticism continues to this day. From 1918 through the Iraq War, “Wilsonian” is the only word in the American foreign-policy lexicon that remains both a proud boast and a cutting insult.

In the interwar era, blame for U.S. entry into the now-reviled First World War was sometimes assigned to the sinister wiles of British intelligence, sometimes to the financiers at the Morgan Bank, sometimes to the profit-seeking of the U.S. armaments industry, sometimes to the inherent violence of capitalism itself. Between 1934 and 1936, a congressional committee chaired by Senator Gerald Nye of North Dakota gained headlines by its investigations of banks, munitions makers, and other so-called “merchants of death.” In their greed, these businessmen and bankers (it was alleged) had unscrupulously duped the good and trusting American people into treading “the road to war,” the title of an accusatory bestseller of the same period.

The former merchants of death got an image overhaul when President Franklin Roosevelt renamed America’s military industries “the arsenal of democracy.” Yet the conspiratorial way of thinking about the origins of wars remained alive. “He lied us into war,” Clare Boothe Luce bitterly remarked of Roosevelt’s policy in 1939-41, although the people who repeated her accusation often overlooked the second half of her sentence, which acknowledged the war’s necessity “because he did not have the courage to lead us into war.”

The conspiratorial account of the First World War was temporarily overwhelmed by the attack on Pearl Harbor. A journalist who lived through both the first and second world wars, Frederick Lewis Allen, marveled that the United States went to war in 1941 “with no enthusiasm and no dissent.” Only cranks now question the necessity to fight and defeat Nazi Germany and Imperial Japan. But as the United States settled into the long Cold War against the Soviet Union, the isolationist themes of the interwar era were periodically revived. War and conflict were unnecessary—imposed by self-interested coteries and cliques upon a United States that otherwise had no reason to fight.

To understand why the U.S. fought in 1917, begin by considering the outcome if the United States had not fought. Minus U.S. reinforcements on land and sea, it’s difficult to imagine how the Allies could have defeated a Germany that had knocked revolutionary Russia out of the war.

By the summer of 1917, the Western Allies had exhausted their credit in U.S. financial markets. Without direct U.S. government-to-government aid, they could not have afforded any more offensives in the West. The exhausted Allies would have had to negotiate some kind of settlement with Central Power forces occupying almost all of what is now Ukraine, Poland, and the Baltic republics in the east; most of Romania and Yugoslavia in Southern Europe, as well as a bit of Italy; and almost all of Belgium and most of northeast France. Even if the Germans had traded concessions in the West to preserve their gains in the East, the kaiser’s Germany would have emerged from such an outcome as the dominant power on the continent of Europe. The United States would have found itself after such a negotiated peace confronting the same outcome as it faced in 1946: a Europe divided between East and West, with the battered West looking to the United States for protection. As in 1946, the East would have been dominated by an authoritarian regime that looked upon the liberal and democratic Anglo-American West not just as a geopolitical antagonist, but as an ideological threat.