The Nimzo-Indian is an extremely popular and reliable defence, upon which most world champions have relied 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 centre. The result is sharp, forcing play of a type that is rare in the early stages of most Nimzo-Indians. 4 f3 is an excellent choice for ambitious players who are willing to prepare carefully, as it can be used to steamroller opponents who have not worked out an accurate response.