Opening Evolution - Philidor Countergambit

LEARNING an opening variation by looking at how strong players handled it over the decades - over the centuries in the case of the Philidor Countergambit - can be very instructive.
It does not matter if you have no intention of playing the variation with either colour - learning something new is helpful to general chess understanding.
George Atwood - Jonathan Wilson
London 1799?
Notes in italics are algebraicised from 500 Master Games Of Chess by Savielly Tartakower and Julius du Mont.
1.e4 e5 2.Nf3 
François-André Danican Philidor in his heyday in the third quarter of the 18th century was head-and-shoulders above his rivals. But he had ideas that lesser players were able to spot as simply being wrong, and one of them was Philidor's insistence that Nf3 on the second move is a mistake. He believed the centre had to be fought for with pawns, so blocking the f pawn handicapped White. No serious player believes that today, although there is a similar belief that Nc3 in d-pawn openings is a mistake because it stops White's c pawn joining the battle for the centre.
2...d6 3.d4 f5!?
The starting point of the Philidor Countergambit. 3...f5!? was also Philidor's answer to other third moves by White. He held it was "very advantageous" to exchange a bishop's pawn for a centre pawn, especially so in the case of the f pawn as the exchange simultaneously half-opens the f file for a castled rook. Modern theory holds that 3...exd4 is Black's best move, but 3...f5!? is still reasonably popular at club level and occasionally turns up among games of the elite (Bronstein, Nakamura, Mestel, Kosten, etc).
4.dxe5 fxe4 5.Ng5 d5 6.e6
Driving an uncomfortable wedge into the hostile position.
6...Nh6 7.Nc3 c6 8.Ngxe4!?
A very early example of the 'positional sacrifice', which is esteemed so highly at the present day [1952].
Stockfish10 and Komodo10 prefer 8.g3!?, with the idea of protecting the e6 pawn with Bh3.
8...dxe4 9.Qh5+ g6 10.Qe5 Rg8 11.Bxh6 Bxh6 12.Rd1 Qe7 13.Bc4

The same opponents reached this position at least five times

*****
*****
*****
*****
*****
13...b5?
Wilson played this twice, as he did for 13...Bg7, which is the choice of Komodo10 and Stockfish10. Wilson also once tried the irrelevant 13...a6? He lost all five games.
14.Bb3
Even stronger is 14.Nxe4! as 14...bxc4 loses to 15.Nd6+ Kf8 16.Nxc8, eg 16...Nd7 17.Rxd7 Rxc8 18.Rxe7 Kxe7 19.0-0, when White has queen, two pawns and a continuing attack for rook and bishop.
14...a5
This furious counterattack affords White a respite of a 'tempo', which he utilises in masterly fashion. 14...Bg7 would therefore have been better.
15.Nxe4!
Well-calculated. Apparently Black only expected 15.a3, after which …Bg7 would still have been possible.
Atwood played 15.a4, after which the engines reckon the position is dynamically balanced, against Wilson in a game in 1798. The text was presumably a prepared improvement, showing how important opening theory was even more than 200 years ago.
15...a4
Tartakower points out that 15...Bg7 could be met by 16.Nd6+ Kd8 17.Qc5, eg 17...Nd7 18.Qxc6 Rb8 19.Nxc8 and wins without appeal. If 16...Kf8, Tartakower gives 17.Qf4+ Qf6 18.Nxc8! Qxf4 20.Rd8#.
16.Nf6+ Kf8 17.Nxg8 Kxg8 18.Rd8+!
A beautiful 'deflecting sacrifice'. and the crowning glory.
18...Qxd8 19.e7+ 1-0