In his 1913 work the Psychology of Revolution, Le Bon lays this out in gory detail..
Among the most important factors of history one was preponderant—the factor of beliefs. How are these beliefs born, and are they really rational and voluntary, as was long taught? Are they not rather unconscious and independent of all reason? A difficult question, which I dealt with in my last book, Opinions and Beliefs.
So long as psychology regards beliefs as voluntary and rational they will remain inexplicable. Having proved that they are usually irrational and always involuntary, I was able to propound the solution of this important problem; how it was that beliefs which no reason could justify were admitted without difficulty by the most enlightened spirits of all ages.
The solution of the historical difficulties which had so long been sought was thenceforth obvious. I arrived at the conclusion that beside the rational logic which conditions thought, and was formerly regarded as our sole guide, there exist very different forms of logic: affective logic, collective logic, and mystic logic, which usually overrule the reason and engender the generative impulses of our conduct.
This fact well established, it seemed to me evident that if a great number of historical events are often uncomprehended, it is because we seek to interpret them in the light of a logic which in reality has very little influence upon their genesis.
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The historians who have judged the events of the French Revolution in the name of rational logic could not comprehend them, since this form of logic did not dictate them. As the actors of these events themselves understood them but ill, we shall not be far from the truth in saying that our Revolution was a phenomenon equally misunderstood by those who caused it and by those who have described it. At no period of history did men so little grasp the present, so greatly ignore the past, and so poorly divine the future.
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The mind of the true Jacobin, at the time of the Revolution as now, was composed of elements which we must analyse if we are to understand its function.
This analysis will show in the first place that the Jacobin is not a rationalist, but a believer. Far from building his belief on reason, he moulds reason to his belief, and although his speeches are steeped in rationalism he employs it very little in his thoughts and his conduct.
A Jacobin who reasoned as much as he is accused of reasoning would be sometimes accessible to the voice of reason. Now, observation proves, from the time of the Revolution to our own days, that the Jacobin is never influenced by reasoning, however just, and it is precisely here that his strength resides.
And why is he not accessible to reason? Simply because his vision of things, always extremely limited, does not permit of his resisting the powerful and passionate impulses which guide him.
These two elements, feeble reason and strong passions, would not of themselves constitute the Jacobin mind. There is another.
Passion supports convictions, but hardly ever creates them. Now, the true Jacobin has forcible convictions. What is to sustain them? Here the mystic elements whose action we have already studied come into play. The Jacobin is a mystic who has replaced the old divinities by new gods. Imbued with the power of words and formulae, he attributes to these a mysterious power. To serve these exigent divinities he does not shrink from the most violent measures. The laws voted by our modern Jacobins furnish a proof of this fact.
The Jacobin mentality is found especially in narrow and passionate characters. It implies, in fact, a narrow and rigid mind, inaccessible to all criticism and to all considerations but those of faith.
The mystic and affective elements which dominate the mind of the Jacobin condemn him to an extreme simplicity. Grasping only the superficial relations of things, nothing prevents him from taking for realities the chimerical images which are born of his imagination. The sequence of phenomena and their results escape him. He never raises his eyes from his dream.
As we may see, it is not by the development of his logical reason that the Jacobin exceeds. He possesses very little logic of this kind, and therefore he often becomes dangerous. Where a superior man would hesitate or halt the Jacobin, who has placed his feeble reason at the service of his impulses, goes forward with certainty.
So that although the Jacobin is a great reasoner, this does not mean that he is in the least guided by reason. When he imagines he is being led by reason it is really his passions and his mysticism that lead him. Like all those who are convinced and hemmed in by the walls of faith, he can never escape therefrom.
So if you're looking for a "Rosetta Stone: that might help you understand what we're seeing in America today, LeBon's The Psychology of Revolution is maybe not a bad place to start.
(And here I'll credit the Anonymous $.99 Amazon E-book on Hypnosis author as the one who turned me on to Le Bon - What precisely was up with that guy is a mystery I have yet to crack).