1. d4 d5 2. Nf3 Nc6 3. Nc3 Nf6 Queens pawn game with four knights. Pretty tame so far
4. Bf4 Be6 Blocks in e pawn, would mean black would need to fianchetto to develop the dark bishop if wanting to castle kingside
5. e3 Qd7 6. Bb5 pin knight to queen to complete whites minor piece development
O-O-O This is the key mistake. On the face of it the move looks logical. If we used Tarraschs system to count development tempo white has four ( one for each minor piece developed ) , and black has six ( one for each minor piece, plus the queen, and two for castling ) . However, note that blacks pieces lack mobility. The king has only one move, the queen only two squares to move to, both rooks only one, the dark bishop none, the light bishop only three squares, one knight is pinned to the queen, and the other knight is his best piece with five possible moves
7. Ne5 The key move to exploit blacks lack of mobility
Qe8 The only other option was Qd6 which leads to Nxc6 and the queen has no safe squares ( qd7 loses the queen to Nxa7+ )

8. Bxc6 Threatens the queen again
bxc6 blocks in the queen and exposes the king to attack. The alternative was blocking d7 with the bishop or knight which would have resulted in the loss of at least another pawn leaving black down a knight and pawn
9. Qd3 preparing Qa6

Ne4 again Bd7 would be better
10. Qa6+ Kb8 11. Nxc6+ Threatens checkmate Qxa7
Qxc6 12. Qxc6 The queen is gone and the rest is downhill

Nd6 13. Bxd6 Rxd6 14. Qc5 a6 15. a4 g5 16. Ra3 Bg7 17. Rb3+ Kc8 18. Qa7 Kd7 19. Rb7 Rc8 20. b4 Rc6 21. Kd2 Rd6 22. b5 Bf6 23. Rb1 axb5 24. Nxb5 Rc6 25. Rxc7+ R6xc7 26. Nxc7 Bxd4 27. exd4 Rxc7 28. Rb7 Rxb7 29. Qxb7+ Kd8 30. a5 Bc8 31. Qxd5+ Ke8 32. Qa8 Kd7 33. a6 Bxa6 34. Qxa6 1-0