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Chess Opening Theory/1. e4/1...e5/2. Nf3/2...d6/3. d4/3...Nd7/4. Bc4/4...Ngf6/5. dxe5
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Hanham variation C41 Philidor defence → Hanham variation → 4...Ngf6
5. dxe5
The critical move. White opens the centre and attacks Black's knight on f6.
The pawn must be retaken.:
Simply moving the knight loses more material to exd6 and/or Bxf7+, e.g. 5...Ng4? 6. Bxf7+ Kxf7 7. Ng5+ Ke8 8. Ne6 (forking Q and B)+++.
Black may think White's e-pawn is hanging and that 5...Nxe4?? kills two birds with one stone. However, 6. Qd5! threatens Qxf7# while forking the knight on e4. 6...Qe7 7. Qxe4+++.
These leaves Black with two ways to take the pawn on e5.
5...dxe5?? is a mistake because White can launch a winning attack f7. 6. Ng5! and White threatens Bxf7+ Ke7 Ne6 and Black's queen is trapped.
6...Nb6 is the best attempt. 7. Bxf7+ Ke7 8. Qxd8+ Kxd8+++ White is up a pawn in an endgame. Bb3 and Nf7+ forking the king, rook and pawn is threatened.
6...Bb4+ at least allows Black to castle if they give up the bishop. 7. c3 O-O 8. cxb4.
5...Nxe5 is Black's best move. It recaptures the pawn and attacks White's bishop, so White isn't in time to play 6. Ng5. However, after 6. Nxe5 dxe5 White is able to win at least a pawn tactically.
Theory table
+5678910dxe5
dxe5?Ng5 +-dxe5
Nxe5Nxe5
dxe5Bxf7+
Kxf7Qxd8
Bb4+Qd2
Bxd2+Nxd2 +/-
References
See also