Cunningham Defense

With 3...Be7, Black develops while reviving the check that 3. Nf3 was meant to prevent. After ...Bh4+, White can lose the right to castle, and if the king's bishop is still on f1, Ke2 shuts it in. Blocking with g3 is unattractive: 4. g3 fxg3 5. hxg3 Nf6 leaves a weak g3 pawn and an h-file of little use. 4. h4, the third most common reply, rules out the check at the cost of loosening the kingside. 4. d4 seizes the center and clears d2 for the king, but Black scores well against it in online play. The engine considers the position roughly balanced.

The main line is 4. Bc4, which vacates f1 for the king and targets f7. Black can still check with 4...Bh4+ 5. Kf1, while 5. g3 fxg3 6. O-O gxh2+ 7. Kh1 is the Bertin or Three Pawns' Gambit, a 19th-century line. Modern practice favors 4...Nf6, the McCormick Defense, when 5. e5 Ng4 is the Modern Cunningham and 5...Ne4 the trappy Euwe Variation. The plain developing move 4. Nc3 allows 4...Bh4+ 5. Ke2, and White keeps a strong center in return for the exposed king.