In Chapter 18 of In the Garden on Beasts" the question is:
At this point in Hitler’s rise to power, no military action was taken to halt his ascension. What would have been the most likely outcome of preemptive military action?
The answer to this can be found on the last page of Chapter 18. There, Larson tells us that any military action taken against Hitler and Nazi Germany at that time would surely have led to the downfall of the regime.
Let us look at the context in which this statement is made. On October 14, 1933, Hitler announced that, in essence, he was going to break practically every commitment that Germany had under the Treaty of Versailles. He was going to pull Germany out of the League of Nations. He was going to pull the country out of disarmament talks that were going on in Geneva as well. He was, Larson says, announcing that he was going to rearm Germany (it had been prohibited by the Treaty of Versailles from having a large military with offensive capabilities).
Larson argues that the Allies could easily have defeated Germany if they had decided to take military action at that point. He quotes William Shirer’s classic The Rise and Fall of the Third Reich, in which Shirer says
That the allies at this time could easily have overwhelmed Germany is as certain is as certain as it is that such an action would have brought the end of the Third Reich…
Thus, Larson is arguing that Nazi Germany could have been destroyed easily at this time and World War II might never have happened.