Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e6/3. Nc3/3...Bb4/4. a3

This move can be seen as the critical test of the Nimzo-Indian; White is willing to spend a tempo to force Black to carry out their plan of neutralizing the c3-knight.

With 4. a3, the Sämisch named after the German Grandmaster Friedrich Sämisch, White immediately questions the placement of the bishop. This has the benefit of seizing the bishop pair early (if Black takes the knight), and resolving central tension. White will play for an eventual e4 push after f3, often transposing to or from variations with 4. f3.

Taking the Knight is forced as the alternatives are illogical or unsafe.

4...Bxc3+ inflicts the doubled pawns on White and is the main continuation.

4...Ba5?? Maintaining the pin is an error as it just loses the bishop after 5. b4 Bb6 6. c5.

Playing 4...Be7?! is better, but it defeats the point of the Nimzo-Indian since White gets to play 5. e4 for free. (The point of the 3...Bb4 pin was to prevent White from doing this easily.)

1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. a3

• Nunn's Chess Openings. 1999. John Nunn (Editor), Graham Burgess, John Emms, Joe Gallagher. ISBN 1-8574-4221-0.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.

Wikipedia has related information at Nimzo-Indian Defence.