In the years after the First World War, American president Woodrow Wilson predicted that if America refused to join the League of Nations, there would be a Second World War.
America did indeed refuse to join the League of Nations; and there was later a Second World War. Thus, it might seem at first glance that he was a prophet, or that World War II really was the result of not joining the League.
But this is a problematic claim for several reasons - others, too, predicted World War II; and their causality claims were somewhat different. John Maynard Keynes, for example, predicted that World War II would happen if the Allies pursued reparations from the Germans; and he had much criticism of the League of Nations advocated by Woodrow Wilson. Even if accurately predicting the war comes from a genuine prophecy (rather than a lucky guess), that doesn't mean that the predictor's reason for why it happened is the true reason - causality is a little more complicated than that.
I'll leave the discussion of causality to another post; and instead focus here on John Maynard Keynes' predictions - because if accurately predicting an event means that someone is right about why it happened, then John Maynard Keynes' predictions would prove Woodrow Wilson is wrong; and I will give the quotes to prove it now.
John Maynard Keynes
Mr. Keynes is best known as one of the towering figures in the history of economics; and wrote a book in 1919 called "The Economic Consequences of the Peace", which accurately predicted World War II. I quote the relevant portion now: "If we aim deliberately at the impoverishment of Central Europe, vengeance, I dare predict, will not limp. Nothing can then delay for very long that final civil war between the forces of Reaction and the despairing convulsions of Revolution, before which the horrors of the late German war will fade into nothing, and which will destroy, whoever is victor, the civilization and the progress of our generation." (Source: Chapter VII, Section 1)
The last German reparations were paid in 1932, the year before Hitler came to power in 1933. The reparations "aim[ed] deliberately at the impoverishment of Central Europe," and sure enough, they were followed by a "civil war between the forces of Reaction and the despairing convulsions of Revolution, before which the horrors of the late German war [would] fade into nothing."
Hitler addressing the Reichstag, 1933
There was much horror from prewar Nazism before that war started, but the beginnings of the aggressive Nazi policy followed 14 years of reparations, which left Germany aching for revenge. Vengeance, indeed, did not "limp" - it came along with terrifying momentum.
Pro-Wilson documentary making his claims of causality
The argument given by President Wilson's admirers seems to be that because Mr. Wilson accurately predicted World War II, his explanation of why it happened (namely, our not joining the League of Nations) was also correct. Yet Mr. Keynes accurately predicted World War II as well, and his explanation of why it happened is somewhat different. Beyond his attribution of such a war to reparations - rather than the absence of a League of Nations - he had much criticism of the League of Nations that Wilson did not share, which may have influenced American public opinion against it.
First meeting of the League of Nations, Geneva 1920
It's true that Keynes agreed with Wilson that the League of Nations "has already accomplished a great and beneficent achievement", that "the wisdom of the world may yet transform [it] into a powerful instrument of peace," and that "our first efforts for the Revision of the Treaty must be made through the League rather than in any other way." But he also said that "the League in the hands of the trained European diplomatist may become an unequaled instrument for obstruction and delay. The revision of Treaties is entrusted primarily, not to the Council, which meets frequently, but to the Assembly, which will meet more rarely and must become, as any one with an experience of large Inter-Ally Conferences must know, an unwieldy polyglot debating society in which the greatest resolution and the best management may fail altogether to bring issues to a head against an opposition in favor of the status quo." (Source: "The Economic Consequences of the Peace," Chapter VII, Section 1)
He further said that "There are indeed two disastrous blots on the Covenant ... which together go some way to destroy the conception of the League as an instrument of progress, and to equip it from the outset with an almost fatal bias toward the status quo." (Source: "The Economic Consequences of the Peace," Chapter VII, Section 1)
Far from President Wilson's romantic conception of the League of Nations - very far indeed.
German demonstrations against Treaty of Versailles, Reichstag 1919
Thus, it would seem that Mr. Keynes' predictions were quite different. He criticized the League of Nations to a degree Wilson did not, and attributed the coming war to reparations - rather than the absence of a League of Nations. If Mr. Wilson's accurate prediction of World War II makes him correct about its causality, then Mr. Keynes' prediction would likewise make Wilson incorrect - destroying any confidence in settling the issue by relying on someone's predictions. They contradict each other to a large degree, and so no one's predictions can alone be taken as vindication for their version of what caused it - or even that their prediction was anything but a lucky guess. Causality is a little more complicated than that.
German troops marching in Paris,
soon after its surrender in 1940
On the other hand, the history of Germany between the wars strongly shows that Keynes' predictions of a German appetite for revenge were well-founded - that was indeed why the Germans went to war, and Keynes' causality is thus borne out by an accurate reading of the evidence, to a degree Wilson's causality is not. Keynes was right about this, and Wilson was wrong.
Woodrow Wilson movie
World War One miniseries
World War Two miniseries