It’s hard to say if some kind of general war would or would not have broken out in (or around) the late 1930s if Hitler had died during or soon after the First World War. If one had broken out, however, it probably would have had major differences, given Hitler’s importance in shaping the course of the historical one.
While we’re on the subject of how history would have been affected if Person X had died in Year Y instead of Year Z, here’s another example. The book I previously mentioned (“What If? The World’s Foremost Military Historians Imagine What Might Have Been”) describes how, sometime around 1888, Buffalo Bill’s Wild West Show performed in Germany during European tour. Annie Oakley, the company’s star sharpshooter, issued (as usual) an invitation for a member of the audience to step on the the stage and allow her to shoot the ash from the tip of his cigar with her revolver. Normally, nobody ever accepted her invitation (thus requiring her husband Frank Butler to serve as her usual target). This time, however, somebody did take her dare: Kaiser Wilhelm II. Annie’s gunshot neatly clipped the end of his cigar, but she had apparently consumed more than her usual amount of whiskey the night before and it’s possible that her bullet might have ended up in the Kaiser’s head. “What if?” speculates on how this might have affected the Anglo-German naval rivalry of the late 19th and early 20th centuries, and other factors which contributed to the outbreak of WWI.
You’ve raised an interesting “What if?” question. While we’ll never have a definitive answer, it’s interesting to look at the historical background and likely scenarios surrounding it.
In The Polish-Soviet War of 1919-1921, no major Western democracy either sent soldiers to Poland or exerted significant diplomatic pressure on the Soviets to back down. As a consequence, Poland came very close to having been annexed outright by the Soviet Union.
Fortunately for the Polish, the Soviet Union was still in a state of civil war during this time. That fact, and the courage of the Polish servicemen, allowed the Polish to win an unexpected victory outside their capital city of Warsaw. Instead of obtaining the best surrender terms they could (as the leaders of Western democracies had advised), the Polish were able to retain almost all their territory intact. This war clearly demonstrated that the Western democracies would not act to contain Soviet expansionism.
The Western democracies generally became more pro-Soviet in the '30s than they had been in the '20s. In 1935, both France and Czechoslovakia signed defensive alliances with the Soviet Union. During most of the '20s the U.S. President had been an isolationist: Calvin Coolidge. But starting in 1933, the U.S. president was a pro-Soviet interventionist: FDR.
Germany lacked the raw materials to adequately supply its factories, and the farmland required to adequately feed all its people. It was a net importer of both food and raw materials. To acquire the money for these imports, it needed to be a net exporter of manufactured goods. Germany could be economically very strong during times of peace and free trade, but was vulnerable to naval blockades and trade barriers.
Major Western democracies took several measures which significantly harmed Germany’s economy. During the late '20s and early '30s Britain and France closed themselves and their empires to German imports. The loss of trade with colonial territories was especially significant because the colonies were a natural trading partner. Germany needed food and raw materials, and could provide manufactured goods. The colonies needed manufactured goods and could provide food and raw materials.
In 1930, the United States also closed itself to most German imports via the Smoot-Hawley Tariff. This was not aimed specifically at Germany: it was a politically motivated renunciation of the concept of free trade.
The punitive reparations payments required by the Versailles Treaty left Germany with a staggering amount of debt. It now had to find the money to service that debt, plus to pay for the large amounts of imports required to feed its people and to provide raw materials for its factories. It had to do this when many of the world’s largest economies had closed themselves to German imports. This situation resulted in an economic catastrophe inside of Germany, and meant that large numbers of Germans were unable to adequately feed themselves or their children.
At this point Germany had three options. Option 1 was to go communist, and there were many within Germany who felt their nation should do exactly that. However, a significant number of refugees from the Soviet Union had fled westward into Germany. The presence of these refugees meant that many Germans were at least dimly aware of the atrocities and mass murders occurring inside the Soviet Union. (Even though these events were generally deemphasized by major media outlets in both Germany and other Western nations.)
Option 2 was to maintain the status quo. It is human instinct to believe that maintaining things as they are is generally a safer and more cautious approach than making a change. Balancing out this instinct was the fact that many German parents had to send their children to bed hungry each night. In addition, the Western democracies did not have any plan to contain Soviet expansionism, or any visible willingness to form such a plan. The Versailles Treaty limited Germany to a token military. The Western democracies had done nothing or next to nothing to stop the Soviet invasion of Poland, and were highly unlikely to act against other Soviet invasions in Eastern or Central Europe. Maintaining the status quo and adhering to the military weakness mandated by the Versailles Treaty would most likely mean that Germany would eventually be added to the USSR.
Option 3 was for Germany to renounce the Versailles Treaty, and to adopt policies which were nationalistic, militaristic, and strongly anti-communist. The theme of this option was that if the Western democracies would do nothing to stop a Soviet invasion of Germany, Germany would become strong enough to resist such an invasion on its own.
Could a German leader have combined a strong military with an anti-communist foreign policy without going to war against either the Soviet Union or the Western democracies? The answer to this question is not immediately clear. A strong German army could be sent either west or east, and it would be very difficult for any German leader to prove that his goal was to stand up to the Soviets, but not to provoke a war with the West. The stronger the German military became, the more suspicious leaders in Britain and (especially) France were likely to become. During the 1930s, diplomatic relations between Germany on the one hand and Britain and France on the other deteriorated because of exactly this kind of suspicion. While the Nazi regime was hardly noted for its diplomatic subtlety, gracefulness, and tact, it is difficult to see how any German government could have entirely allayed Western democratic suspicions while also maintaining the military strength necessary to resist a Soviet invasion.
No matter who Germany’s leader during the '30s and '40s had been, it is almost impossible to see how he could have both a) avoided a major war, and b) avoided Soviet occupation of Germany.