Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e6/3. Nc3/3...Bb4/4. f3

4. f3 enters the aggressive, and increasingly common, Kmoch Variation of the Nimzo-Indian Defense. White moves their pawn to f3 with a very clear idea; to advance with e4 and gain a massive pawn center, suffocating Black's army. This is one of the most aggressive and dangerous weapons for Black to face, and Black needs to abandon some of their key plans in order to combat the serious threat behind the f3 move.

With the plan of f3, e4, and eventually e5 to scare away Black's king knight, White gains a colossal pawn center. However, the Kmoch Variation carries several risks. f3 is not a developing move; in fact, it's move that weakens White's kingside shelter and risks creating holes near White's king which Black could exploit later on. Furthermore, White's colossal center can be struck at by Black's pawn breaks, potentially leaving White's center flimsy and vulnerable to attack. Black has several ways of countering the Kmoch Variation.

4...d5, in similar fashion to the Queen's Gambit Declined, clamps down on the e4 square. With this approach, Black looks to stop White's e4 move at all costs, hindering White's central expansion.

4...c5, in similar fashion to the Benoni Defense, strikes at White's center with a flank pawn. With this approach, Black forces White's center to overextend with 5. d5 and can even undermine the center further with b5 at some point. Note how this approach allows White to play the move 6. e4; Black looks to demonstrate that White's overextended center will become a huge target for Black's superior piece development.