Chess Opening Theory/1. e4/1...e5/2. Nc3/2...Nf6/3. f4/3...d5

=Vienna Game= Black's counter-attack leaves White with few options. White is almost forced to take the e-pawn: /4. fxe5/.

/4. d3/ is hard to refute but it gives initiative to Black. This move is usually played to strengthen the center, but in this case, Black has many ways to break it down easily. The threat Qxd1 Kxd1 prevents White's d-pawn from protecting the king's pawn effectively. Also note that White's f-pawn is no longer protected by the e5 trap, because Black can reply with Ng4 or d4.

/4. exd5/ is not advised since it will dramatically weaken White's pawn structure (doubled pawns on the d-file and the d5-pawn is en prise). In this case, Black should not immediately take back the d-pawn with 4...Nxd5 but rather try to exploit the weakness in White's pawn structure with 4...e4 or 4...exf4.

Theory table
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1. e4 e5 2. Nc3 Nf6 3. f4 d5