“First; he was over confident from past successes. 2nd; he was very ambitious and had presidential ambitions”. Quote: feralmerril

The ego and ambition undoubtedly led to seeing things the way you want them to be rather than the way that they are.
He was very successful and lucky during the civil war, Politics (the need for successful commanders) had a part in his promotion to Brevet Brigadier General at a very young age. When you read about his post war conduct it is hard to escape the notion that the man was very full of himself.

All the other points above about the unique situation he faced are undoubtedly true, but still we have a commander who looked at the situation and saw what he wanted to see. In life things are the way they are, seldom the way we wish they were.

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Ipsis Rebus Dictantitbus