The Halosar Trap (named after Hermann Halosar) is a chess opening trap in the Blackmar–Diemer Gambit.
1. d4 d5 2. e4?!
- This is the start of the Blackmar–Diemer Gambit.
2… dxe4 3. Nc3 Nf6 4. f3 exf3
- Now 5.Nxf3 is usual, but by capturing with the queen, White lays a trap. (This is the Ryder Gambit.)
5. Qxf3?! Qxd4 6. Be3 Qb4?!
- Better is 6…Qg4. Black thinks that castling is prevented because of …Bg4, but White castles anyway.
7. 0-0-0 Bg4?? (see diagram)
- Blundering into the trap.
- White threatens mate with 9.Nxc7#. The black queen cannot capture the knight because 8…Qxb5 9.Bxb5+ is check, gaining time for the white queen to escape the black Bg4’s attack.
8… Na6 9. Qxb7 Qe4
- Black lost even more quickly in Diemer–Halosar, Baden-Baden 1934, after 9…Rc8 10.Qxa6 1–0.
10. Qxa6 Qxe3+
- Worse is 10…Bxd1 11.Kxd1 Rd8+ 12.Bd2 and White is winning, for example 12…Ng4 13.Nxc7+ Kd7 14.Qxa7.
11. Kb1 Qc5
- The White threat of 13.Qb7 wins the black a-pawn by force. With even material and a passed a-pawn, White will have a winning advantage (Burgess). Even stronger seems 12.Qb7! with the idea 12…Bxd1 13.Qxa8+ Kd7 14.Nc3 and White has a winning attack.