George B. McClellan is your answer
McClellan was a prominent general towards the beginning of the civil war, and was given the Army of Pontomac that was placed under his control. He was able to train them to become one of the largest US armies that was well-equipped, well-trained, and was a formidable force. However, McClellan was known to many of the US government officials as a "coward". He failed to move his army out to war many times, and was afraid of losing. He also always over-estimated the enemies, and started to have a harrowing relationship with Lincoln, leading to his removal from being the leader of the Army of Pontomac. He later ran against Lincoln in the Democratic Party before Lincoln's second term.
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