The Nimzo-Indian is an extremely popular and reliable defense, upon which most world champions have related at some point in their careers. Black controls the vital e4-square with his pieces, and retains total flexibility with his central pawns. By playing 4 f3, White challenges Black's idea head-on. Either Black grants White control of e4, or else he must commit his pawn to d5, whereupon White can begin concrete action in the center. The result is sharp, forcing play of a type that is rare in the early stages of most Nimzo-Indians. The 4 f3 Nimzo enjoyed a period of great popularity in the early 1990s, following a number of spectacular victories by the teenage Alexei Shirov. However, good responses were found and the line's popularity declined. A number of players quietly worked away at revitalizing 4 f3, and it is now once again a potent weapon and will be greatly valued by club players wishing to spring 4 f3 on their opponents.