Sergeant Skinner (born Walter Seymour Skinner in Springfield) was a member of the U.S. Army. He dreamt of becoming principal of the Springfield Elementary School. He was five years in a POW camp, then has been sold to a shoe-producing company in China, and released when the United Nations closed the company. He then returned to Springfield, where he tried to take his life back.
The Real and the Fake
Skinner is the original Seymour Skinner - Springfield Elementary School's principal is actually named Armin Tamzarian. The real Skinner and Armin Tamzarian were buddies in the Vietnam War. Skinner took Tamzarian under his wing and turned him from a rebel into a responsible adult. Skinner would later disappear on a mission. Tamzarian, assuming Skinner was dead, decided to report the death to Skinner's mother. However, she thought Tamzarian was Skinner, who not wanting to sadden her, played along. In the end, the citizens of Springfield rode him out of town on a rail, in favour of Tamzarian, who was then legally renamed Seymour Skinner. The "real" Seymour Skinner has not appeared again.
All the events were forbidden from being mentioned again, under penalty of torture. The only time this rule has been broken since was when Lisa refers to Principal Skinner as "Principal Tamzarian". Also, a tombstone reading "Tamzarian" can be seen in Homer's fantasy about his own funeral in The Wizard of Evergreen Terrace.
Behind the Laughter
When Lisa called Principal Skinner Principal Tamzarian, this was a self-referential joke, since "I, D'oh-Bot" used the same plot device of making a major change to the Simpsons' world, then effectively erasing the change within the same episode.