After directing the original Star Wars, George Lucas stopped directing movies for more than twenty years. He stayed active in Hollywood, working as a producer and devising stories that other directors would tackle, such as the concept for Indiana Jones, which Steven Spielberg would direct. The streak ended in the late 90's when Lucas returned to the director's chair to create The Phantom Menace.
But George Lucas wasn't George Lucas's first choice of director. According to Ron Howard, Lucas asked Robert Zemeckis, Steven Spielberg, and Howard himself to direct the films, but all three filmmakers turned down the honor—with Howard speculating they did so because the task of following up the original trilogy would have been too daunting. (Source)
Whatever the case, Lucas eventually accepted his own offer. Picking up where he left off, he seems to have set out to accomplish two things. First, he wanted to show audiences a previously unexplored part of the Star Wars saga. Second, he wanted to revolutionize special effects in the film industry. And he accomplished both.
By serving as the writer as well, Lucas was able to tell the story of Anakin Skywalker the way he wanted to. Working with Industrial Light and Magic, he also created a film that was a watershed in special effects. With 2,000 digital visual effects, "only 12 minutes of the 133 minute film had no special effects." For comparison's sake, Return of the Jedi had only 900 visual effects shots. (Source)
Of course, today entire films are nothing but digital effects, but for 1999 Lucas's directorial ambitions resulted in quite the achievement, and it still looks pretty good today.