Tension builds up. Black is threatening Qh4+ which will be devastating. Therefore, White needs to be quick. However, White has two center pawns to Black's one, which helps central control.

A common mistake is to immediately take the black knight with 5. Nxe4. After 5...dxe4, White has too few options to pursue his development; the f3-square is under control, the queen's bishop is blocked and the d-pawn cannot advance safely. Moreover, it is going to be hard to defend the e5-pawn against Nc6. It looks possible to catch up with 6. d4 but it seems that Black has equalized.

5. Nf3 looks like the most solid move. It defends the e5-pawn (who is quite fragile now that he is away from home) and secures the strategic d4-square. It also prevents the Qh4+ threat.

5. d3 is a good way to deal with the problem quickly. Black's knight will have to retreat or, more likely, to be traded with White's knight. White should be aware that Black can launch an aggressive attack with Bb4 and Qh4, sacrificing the knight, but preventing castling after 5...Bb4 6. dxe4 Qh4+ 7. Ke2 (7. g3 is bad because the white knight is pinned: 7...Qxe4+).

5. Qf3! is best, threatening to win a pawn by Nxe4 dxe4 Qxe4, as well as covering the Qh4+ threat. However, even still, Black has ways to obtain easy equality, or go into an unclear position where both sides have chances.

For explanation of theory tables, see theory table and for notation, see algebraic notation..

1. e4 e5 2. Nc3 Nf6 3. f4 d5 4. fxe5 Nxe4

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Wikipedia has related information at Vienna Game