Answer:
When Grant applied for west point, the congressman recommending him forgot Hiram's middle name and thought it was his mother's maiden name, so he put Ulysses Simpson Grant. When Ulysses arrived and found out the mistake, instead of going to all the trouble to resubmit the papers, he resigned his name, as short as possible: US Grant. His nicknames he received later were Uncle Sam and Unconditional Surrender.