If it was not hardships resulting from the treaty of 1919 that caused Germany to go to war again in 1939, what was the actual cause?

A part of the cause was undoubtedly the business depression that started in the late 1920's. All countries suffered from this depression, the United States as much as any—and Germany less than some other countries. Everywhere there were bank crashes, business failures, and millions of workers unemployed. In no other countries, except Japan and Italy, did this lead people to think of war as a remedy. But in Germany certain groups that wanted another war used the depression to make many of the people believe that their sufferings were due to the malice of the countries which had defeated them in World War I. The fact that those countries were suffering equally was disregarded. The troubles of the Germans were blamed largely on reparations payments—though no reparations were being paid, or had, in reality, ever been paid, except out of money advanced by the Allies. By propaganda on an immense scale, the misery and discontent resulting from the depression were turned into hatred of other countries. A belief was fostered that the way of escape for Germany would be through extending its territory and increasing its wealth at the expense of those other countries—if need be, by war.

But long before the depression, and years before Hitler came to power, there were two groups of Germans who were determined not to accept Germany's defeat in the first World War as final. These were, first, the officers, especially the higher officers, of the old professional army, and second, the heads of some of the big industrial organizations and trusts, especially the munitions manufacturers. These worked together, and their platform (“Pan-Germanism”) even before the first World War demanded the expansion of Germany's power and territory by force of arms. The defeat of 1918 did not cause them to give up their program; they believed the defeat was only a temporary setback.

These groups, however, had no very large support among the German people until the National Socialist (Nazi) Party was formed. This party was founded and headed by men with great skill in political organization and in arousing resentful feelings among the masses. The military group and some of the industrial magnates, who provided money for the Nazi party's political campaigns, thought to use it as a tool. Hitler himself was at first a paid secret agent of a clique of army officers, though in the end he became far more powerful than his original backers and far more ruthless.

But to understand fully what happened in Germany in these twenty years, and especially in the 1930's, it is necessary—many students of German history think—to understand certain ideas and feelings which influenced many Germans long before Hitler's regime, but for which he became the most effective propagandist.

One of these is the belief that the Germans are a superior race, the “master race” of the world, who have a right and a duty to extend their power and impose their ideas and their kind of civilization upon all other and “inferior” peoples. Especially after the last war, this belief, preached by hundreds of writers and orators, gained an enormous hold upon the German mind—the more easily because it gave relief from the sense of inferiority and the humiliation which had been caused by defeat. All who accepted it were bound to look forward to another and a bigger war—a war for world supremacy—as necessary and desirable.

“Today Germany belongs to us,
Tomorrow the whole world”

the Hitler Youth sang in one of their marching songs.

It was largely by playing upon these ideas and feelings and by arousing hatred against Jews, bankers, Communists, German believers in democracy, and all democratic foreign countries, that the Nazi party got great masses of the German people to support it, so that by 1933 it was the largest single political party in Germany. In that year the German people were in part persuaded and in part terrorized into giving Adolf Hitler absolute power. That he would in time use this power to start another war, many Germans who could read, or who had heard the speeches of Hitler and other Nazi orators, must have realized.

From EM 10: What Shall Be Done about Germany after the War? (1944)