Defensive mistakes can be fatal

Chess Checks

 However the defence of the king at any time in a game takes precedence over any attack and even the slightest lapse can bring the game to an abrupt halt with a loss. In the game which follows, White is in a good position for most part but just when it appears that he is treading the path to victory, he overlooks the king’s defence and ends on the losing side.

White: Victor Mikhalevski (2545) - Black: Valery Loginov (2500)
St Petersburg, 1998
King’s Indian Defence
1.d4 Nf6 2.c4 g6 3.Nc3 Bg7
The King’s Indian defence which is one of the sharp and popular Opening against the Queen’s Gambit
 4.e4 d6 5.h3 0–0 6.Bg5 Na6 7.Bd3 e5 8.d5 c6 . Black would like to settle things in the centre
9.Nge2 cxd5 10.cxd5 Nc5 11.Bc2 a5 12.a3 a4 13.0–0. If White plays aggressively with 13.g4 Bd7 14.Kf1 (14.f3 Qb6 15.Rb1 Rfc8 16.Be3 Qa5 17.Kf2 b5 18.Na2 Na6)14...b5 15.f3 Qb6 16.Na2 h6
13...Bd7 14.g4. White plays a new move here. Previously played was  14.Qd2 Qb6 15.Rab1 Rfc8 16.Ng3 Qd8 27
14...h5. Black also does not shy away from proclaiming war on the King side
15.f3 b5. Now comes another interesting advance on the queen flank. If 15...Qb6 16.Kg2 Qxb2 17.Rb1 Qxa3 18.Bc1 with advantage for White
16.Na2. If 16.Kg2 Nxg4
16...Qb6 17.Kg2
If 17.Be3 hxg4 18.fxg4 Bxg4 19.hxg4 Nxg4 20.Bf2
17...Rac8 18.Nb4. Worth a look is 18.Be3
 18...Na6 19.Qd2
Better appears 19.Nxa6 Qxa6 20.Qd2
19...Nxb4 20.axb4 Nh7 21.Be7
Deciding to enter headlong into enemy territory. If 21.Be3 Rxc2 And if 21.Bh4 Bh6 22.Qxh6 Rxc2 23.Rf2 Rxb2 24.gxh5 g5 25.Bg3 Qd8
21...Rfe8. If 21...Bh6 22.Qxh6 Rxc2 23.Rae1 (23.Rf2 Re8 (23...Rxb2 24.gxh5 Bxh3+ 25.Kg3 And if 23...Rfc8 24.gxh5 Bxh3+ 25.Kg3
22.Bh4 Bf6.
If 22...Bh6 23.Qxh6 Rxc2  24.Rae1 Qd4
23.Bxf6 Nxf6 24.Bd3 Kg7 25.Rf2
With the intention of Doubling rooks on the ‘f’ file. If  25.g5 Nh7 26.f4 Qd8 (26...exf4 27.Qxf4 Rf8 28.e5 Rce8 29.exd6 Bxh3+) 27.h4 exf4 28.Qxf4 Qe7
25...h4 26.Raf1
If 26.Qg5 Qd8 27.Qxh4 Nxg4 28.Qxd8 Ne3+ with advantage for Black
26...Nh7 27.f4. A good move!
27. ..exf4 28.Qxf4. If 28.Nxf4 Re7
28...Rf8. If 28...f6 29.g5
29.e5 dxe5. Black can also consider 29...Qb7
30.Qxe5+ Kg8
30...f6 can also be considered
31.Rf6
The difference between a winning position and a lost one is just this one move! White blunders in time pressure. White is distinctly better after 31.Qe7 Qb7 32.32.Rxf7 Qxd5+ 33.Be4 Qxe4+
31...Nxf6 32.Rxf6 Rce8
If 32...Rfe8 33.Qxe8+ Rxe8 34.Rxb6
33.Qg5. If 33.Qxe8 Qxf6 34.Qxd7 Qxb2 and Black is winning
33...Rxe2+ A counter sacrifice by Black which tilts the game in his favour. If  33...Qe3 34.Rxg6+ fxg6 35.Qxg6+ Kh8 36.Qh7 checkmate
34.Bxe2 Qd4
Diagram 1
35.Rd6. Leaving the King defenceless. This is a bad move as he could offer resistance only with 35.Rf2 Qxb2 36.Bxb5
35...Re8 36.Kf1 Qe4 37.Rxd7
If 37.Qd2 Qh1+ 38.Kf2 Qh2+
37...Qxe2+ 38.Kg1 Qe1+ and White resigned as 38...Qe1+ 39.Kg2 Qg3+ 40.Kh1 Re1 checkmate.  0–1
Diagram 2
White to play and win
1.Qxb4 axb4 2.Rxa8+ Be8 3.Bxd5 Qxd5 4.Rxe8 checkmate.