What's so bad with this pawn advance?

Question

Engines mark White's pawn advance 16. h4 as a 3-point mistake. While I understand that it's going to cost White their g-pawn, which will make White's pawn attack against the castled king much harder, I still don't see why it's so bad as to be a 3-point mistake. Very often losing a pawn is not penalized so harsh. What's the positional or long term disadvantage? The two isolated pawns on f and h?

[FEN ""]
1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. Qe2 Qe7 6. d3 Nf6 7. Nd4 Qxe2+ 8.
Bxe2 g6 9. Bg5 Bg7 10. c3 O-O 11. Nd2 Bd7 12. Bxf6 Bxf6 13. Ne4 Bg7 14. O-O-O
Nc6 15. Nxc6 Bxc6 16. h4 f5

Answer 1

You played a very quiet and unambitious opening, but you were still dreaming of a King-side attack. You might read Vukovic The Art of Attack in Chess and consider his assertion that a successful attack depends on certain "preconditions" being met. You had none of those preconditions, and you should have been focussed merely on maintaining a sound, active position without weaknesses.

I do not understand why you played 12.Bxf6, giving up a Bishop, probably just to play the slightly-aggressive looking 13.Ne4 that achieved nothing. You could have just castled (on either side) put your B on f3 and moved your Rooks to the e-file. The resulting position has very little tension and would be completely equal. By playing 5.Qe2 you gave up on the possibility of building an early initiative.

Even after 12. Bxf6 your position is OK, perhaps a little worse, but 16.h4 is truly a terrible move. After this, you are a Pawn down, you have two weak Pawns, zero compensation and you have opened up the position to your opponents Bishops. You try to shrug this off by saying, in effect, "so what, its just a Pawn", but this is NOT a position where such nonchalance can be justified. 16. Bf3 would have kept the position roughly equal, neutralizing one of those Bishops. The Pawns would be completely balanced (not a passed Pawn in sight) and you have no difficulty mobilizing your Rooks, so it should not be hard to get a draw, but after your previous play a draw should have been the limit of your ambition. You had no right to expect more and the move h4 should never have entered your head.

ADDITION In response to a request from @Mast I quote from an online review from chess.com. You can find this and other reviews via Google.

A large part of Art Of Attack is devoted to what Vukovic considers to be the preconditions necessary for an attack against the castled king to proceed. Among these are the attacker's control of the center, prevention of counterplay on the side of the board opposite the opponent's castled king, proper posting of the attacker's pieces, and a weakening of the castled King's defenses. It is also important that the would-be attacker not commit to the attack until all the preconditions are met, and Vukovic writes at some length, with examples, about what happens when the attack begins too early or too late. Too early and the attack will fizzle, while if the attack begins to late, the attacker may have missed his window of opportunity.

Answer 2

As you infer, losing a pawn in opposite-side castling positions can sometimes be a good thing. Indeed one can sacrifice material in exchange for piece activity, especially if it weakens your opponent's King!

[FEN "r2r2k1/pp3pp1/4pb1p/8/2q2P1P/2P2N2/PPQ3P1/1K3R1R w - - 0 0"]
[White "Smyslov"]
[Black "Rudnev"]

1. g4

Here Smyslov can play 21. g4, apparently giving up the f4 pawn, because if Black captures that White can use the exposed Queen to gain sufficient tempos for a dangerous attack:

[FEN "r2r2k1/pp3pp1/4pb1p/8/2q2P1P/2P2N2/PPQ3P1/1K3R1R w - - 0 0"]
[White "Smyslov"]
[Black "Example"]

1. g4 Qxf4 2. g5 Be7 3. Nd4 Qe3 4. Rf3 Qe5 5. Rhf1 Rf8 6. gxh6

Note that important gxh6 at the end. If you can't crank open the enemies King position then their King safety is fine - and your pawn sacrifices will be for nothing! What's the follow up to your pawn-advance, though? How is White going to open up the Black's King position?

[FEN "r4rk1/ppp3bp/2bp2p1/5p2/4N2P/2PP4/PP2BPP1/2KR3R w - - 0 0"]

1. Ng5 Bxg2 2. Rh2 Rfe8 3. Kd2 Bd5 4. h5

Here it is actually Black's pieces that are coordinating, and Black's pieces that are getting the activity. I know this is spurious, but Black's Bishops rule this board:

[FEN "r3r1k1/ppp3bp/3p2p1/3b1pN1/7P/2PP4/PP1KBP1R/3R4 w - - 0 0"]

1. h5 Be5

White's attack has left their pieces scattered. Black has centralized and looks forward to taking the initiative.

So, what's wrong with your pawn-advance? Well, let's list the problems:

1. It loses the g2 pawn.
2. It increases Black's piece activity.
3. White loses the potential of a later King-side attack.