Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e5/3. dxe5/3...Ne4/4. a3

The move 4.a3 allows White to avoid the annoying bishop check on b4, the also annoying knight jump to b4, and prepares Qc2 to undermine Black's knight. Both Lalic and Nick de Firmian consider it to be White's best move, with de Firmian assessing it as leading to a large advantage for White. Black has several possible answers:


 * De Firmian gives 4...Nc6 as the main move, Black hoping for White to help their development with 5.Nf3 d6 6.exd6?! Bxd6 when Black gets short-term hopes of winning the white queen (e.g. 7.g3?? Nxf2! 8.Kxf2 Bxg3+ ) and long-term hopes of an attack. A game saw 7.e3 Bf5 8.Be2 Qf6 9.O-O O-O-O 10.Qb3 g5! with "a kingside attack that has every chance of success due to White's dormant queenside". Another try was 7.Nbd2 Bf5 8.g3?! Bc5! 9.e3 Qf6 10.Bg2 O-O-O 11.O-O h5! with a "potent kingside attack". Much better for White is the continuation 5.Nf3 d6 6.Qc2! (considered by Lalic to be "the most ambitious move and one that is highly annoying from Black's point of view, as the knight on e4 is forced to declare itself" ) 6...Bf5?! 7.Nc3! Nxf2 8.Qxf5 Nxh1 9.e6 fxe6 10.Qxe6+ Qe7 11.Qd5 h6 12.g3 g5 13.Bg2, when White had a large advantage in the game Reshevsky – Bisguier (New York 1955). Instead, after 5.Nf3 d6 6.Qc2 d5 7.e3 Lalic points out the untried 7...Bg4!? with the possible continuation 8.cxd5 Qxd5 9.Bc4 Qa5+ 10.b4 Bxb4+ 11.axb4 Qxa1 12.Qxe4 Bxf3 13.gxf3 Qxe5.


 * 4...Qh4 (awarded a "?!" by Lalic) was played in the game O'Kelly – Bisguier (San Juan 1969) and Black succeeded in achieving a quick draw, but later developments have called the move into question as Black's development will be lacking. After 5.g3 Qh5 6.Nf3 Nc6 (so far as in O'Kelly – Bisguier), White can play 7.Qc2! attacking the maverick knight. Borik then gives "7...Qf5! (threatening ...Nxg3)", but in fact 7...Qf5?? is refuted by 8.Nbd2! Nxg3 9.e4! and White wins. White has also reached successful prospects by returning the pawn for development, e.g. 5.g3 Qh5 6.Bg2 Qxe5 7.Qc2 Nf6 8.Nf3 Qh5 9.Nc3 Be7 10.h3 c6 11.e4 d6 12.b4 Nfd7 13.Ne2! with the threat 14.Nf4 winning the white queen.


 * Another possibility is 4...d6, hoping for the tactical trick 5.exd6 Bxd6 6.Nf3?? Nxf2! 7.Kxf2 Bg3+ and Black wins White's queen. Stronger is 5.Qc2, when 5...Nc5 is advised as 5...Bf5 is strongly met by 6.Nc3! (playing on the pin of the e4-knight, as 6...Ng3? is rebuffed by 7.e4! ) and 5...d5 would lose a tempo as the d-pawn has reached the d5-square in two moves instead of one. The less critical 5.Nf3 is also possible.


 * A fourth line for Black is 4...b6 (awarded a "!?" by Lalic, who recommends it as the best answer to 4.a3), which is sometimes called the Bonsdorf variation. White cannot win a piece with 5.Qd5?! because of the usual trick 5...Nc5! 6.Qxa8? Bb7 7.Qxa7 Nc6, trapping White's queen. Another point is that Qd1–c2, so effective in most of the other lines, can be met by Bc8–b7. After 5.Nd2 Bb7 6.Qc2, Lalic gives 6...Nxd2 7.Bxd2 a5!, when the black bishops will be excellently placed on b7 and c5. Lalic recommends 6.Nf3 instead, while MCO-15 continues by 5.Nf3 Bb7 6.Nbd2 Qe7 7.Qc2 with a large advantage for White.


 * The slow 4...a5 (awarded a "?!" by Lalic) is a luxury that Black cannot afford, e.g. 5.Nf3 Nc6 6.Qc2 d5 7.e3 Be6 8.Be2 f6 9.exf6 Qxf6 10.O-O Nc5 as in the game Yrjola – Bellon Lopez (Helsinki 1991).

Theory table
1.d4 Nf6 2.c4 e5 3.dxe5 Ne4