Chess Opening Theory/1. d4/1...f5/2. Bg5/2...Nc6

A rare sideline in Hopton attack. Black did not wish to develop their king's knight, 2...Nf6, because White would be happy to trade it off, so instead they developed their other knight. Usually in the Hopton attack, Black doesn't play ...Nc6 because they want to support d5 with c6 (the main line is 1. d4 f5 2. Bg5 g6 3. Nc3 Bg7 4. e3 d5 5. h4 c6). Though White is better by engine, the inaccuracy is not immediately punishable by White.

3. e3 unleashes the bishop and queen, and Black has missed their chance to play ...h6, as 3...h6?? loses to 4. Qh5+! g6 5. Qxg6#. Black can continue with 3...g6 & 4...Bg7 and adopt a Leningrad-style system. Black delays playing ...Nf6 until they may retake with the bishop, so as to avoid Bxf6 exf6 and doubled pawns. For White, the maximalist approach is c4 and Nc3.

3. Nf3 may be followed by 3...h6, and White's bishop really is in danger of being trapped this time because after 3...h6 4. Bh4?? g5 5. Bg3 f4, 6. e3? no longer gives the queen access to h4. White must retreat the bishop, 4. Bd2, and Black has relieved its pressure on their position.

The earliest recorded game of 2...Nf6 is Vesely vs Florian, 1961.^[1] It is a very uncommon sideline. Its highest level adopter is Soviet-born English grandmaster Alexander Cherniaev (1969-).^[2]

1. d4 f5 2. Bg5 Nc6

Wikipedia has related information at Dutch Defence.