3. ... Bg4?
Black's third move is often cited as the first mistake in this game, though it was commonly played at the time. Black will lose time in order to avoid losing a pawn, which allows White to gain the initiative and a lead in development.
4. dxe5
White happily grabs a pawn, knowing that Black cannot recapture with 4. ... dxe5, as this loses a pawn due to 5. Qxd8+ Qxd8 6. Nxe5. Black has another option, however.
4. ... Bxf3
By first capturing the knight, Black threatens White's queen, forcing an immediate response if White doesn't want to lose material.
5. Qxf3
White finishes the trade of knight for bishop. Taking with the queen develops a piece and is superior to gxf3, which would ruin White's kingside pawn structure without developing anything.
5. ... dxe5
Black now recaptures the pawn, bringing back material equality.
6. Bc4
White develops another piece, giving him two developed pieces to Black's none. In addition, White is developing with threats; he is threatening to play Qxf7#, a threat Black must answer.
6. ... Nf6?
This seems like a logical way to defend against the checkmate threat. Black develops a piece while preventing the White queen from being able to reach the f7 square. However, White has a very strong reply.
7. Qb3
This takes us up to our next diagrammed position.
