Which Union general earned his nickname by being a stubborn negotiator?

Eighteenth U.S. President Ulysses S. Grant [April 27, 1822-July 23, 1885] was born Hiram Ulysses Grant. But he was sensitive about the initials: H.U.G. Once away from home, he tried to change his name to Hiram Ulysses. West Point wouldn't allow it. But West Point did allow him to drop the Hiram, and add an S. as a middle initial after Ulysses. The resulting initials brought on two famous nicknames. Throughout his life, the future General and President was described as someone who went straight to the heart of a matter, and was most determined once he set his mind to something. For example, he graduated at the bottom of his class at West Point. But while he was there he tamed York, a horse so wild that the staff would have put him down but for the future Union General and President. The determined cadet simply gave the lifesaving challenge to the Academy: there's no horse that I can't ride. In fact, the young cadet went on to set a high jump record, on York, that remained unbeaten for 25 years. And while he was there, his nickname was Sam, and Uncle Sam. During the War between the States, General Grant was given yet another nickname, based on his initials. For he became ka U.S. and Unconditional Surrender Grant. For he demanded with each Union victory a complete and unconditional surrender.