Kasparov-Petrosian Variation

After 4. a3 Bb7 5. Nc3, White has completed the setup the a3-move was meant for: with ...Bb4 ruled out, the knight supports a future e2-e4, and Black must decide how to contest that square. The same position also arises from 4. Nc3 Bb7 5. a3. The engine considers the game roughly balanced, and White scores slightly better in the database.

The most common answer is 5...d5, the Main Line, which stakes a direct claim to the center. After 6. cxd5 Black chooses between 6...exd5, the Classical Variation, and the more frequent 6...Nxd5, the Modern Variation, where White has several named seventh moves: 7. Qc2 (Kasparov Attack), 7. e3 (Petrosian Attack), 7. Qa4+ (Rashkovsky Attack), 7. Bd2 (Romanishin Attack) and the pawn offer 7. e4, the Polovodin Gambit. Almost as popular is 5...Be7, the Marco Defense, which delays the central decision and keeps a flexible, Hedgehog-like posture. The Andersson Variation, 5...Ne4, trades or blockades the c3-knight at once and gives Black good results, while 5...c5 and the rare 5...g6 are less frequent alternatives.