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

=Nimzo-Indian Defence, Sämisch Variation =

Sämisch Variation
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 his 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.

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.)

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