Chess Opening Theory/1. e4/1...Nf6/2. e5/2...Nd5/3. d4/3...d6/4. Nf3

White develops a piece and prepares to recapture on e5 with their knight. Black still lacks space. They must avoid passive play at all costs, which would allow White to crush them quickly.

4...dxe5, the Larsen variation, resolves the centre tension immediately. White can retake 5. Nxe5, and from e5 they have a powerful knight that cannot be easily dislodged.

4...Bg4 is the main line. This pins the knight and reduces White's control over the centre.

If 5. h3, the Panov variation, Black is happy to trade 5...Bxf3 6. Qxf3 dxe5 (simultaneously defending the knight on d5) 7. dxe5 e6 ⩲, material is equal but Black has weakened White's centre.

The main move is 5. Be2 to unpin the knight, and the idea of 5...Bxf3? in this line fails because 6. Bxf3 dxe5? 7. c4 Nb6 8. Bxb7 winning the exchange +-. 5...dxe5 6. Nxe5 Bxe2 7. Qxe2 ± gives White a slightly superior version of the Larsen variation, and it's difficult for Black to make progress developing their position. White has ideas like Qb5+ to fork the b7 pawn (e.g. 7...e6 (to develop the bishop) 8. Qb5+). In the main line Black leaves the tension and makes modest moves to develop their position in the available space, continuing 5...e6 6. O-O Be7 7. c4 Nb6 8. Nc3 O-O.

4...g6, the Alburt variation, leaves White's pawn chain alone for the time being, and focuses on developing the king's side instead. 5. c4 is playable, and an exchange Alekhine position may be reached (as in 1. e4 Nf6 2. e5 Nd5 3. d4 d6 4. c4 Nb6 5. exd6 cxd6 6. Nf3 g6), but the main move is 5. Bc4, and after 5...Nb6 6. Bb3 Bg7, White makes a number of threats on the king's side to induce weakness. 7. Ng5 (threatens Nxf7) e6 8. Qf3 (threatens Qxf7#) Qe7 9. Ne4 (prepares Bg5).

• 4...Nb6!?, where Black moves their knight back without needing to be prompted by c4, is called the Schmid variation.
• 4...Nc6!?, simply developing their other knight, is the Larsen-Haakert variation.

'1.e4 Nf6 2.e5 Nd5 3.d4 d6 4.Nf3'

Wikipedia has related information at Alekhine's Defence.

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