Chess Opening Theory/1. e4/1...e5/2. d4/2...exd4/3. c3/3...dxc3/4. Bc4/4...cxb2/5. Bxb2/5...Nf6

= Danish Gambit Accepted, Classical Defence =

5...Nf6
Black's knight on f6 is targeting the undefended e-pawn. Turning the tables with 6. e5 is an obvious and strong choice.

Defending the pawn with 6. Nc3 or, if you don't want to block the bishop's diagonal, with 6. Nd2 are less popular. Although developing pieces can't be bad, if Black finds the accurate ...d5 in response then the pawn is getting blown up anyway.

6. Nf3 is just a bit too normal to be an optimal move in this crazy line. The moves Black needs to find in response are correspondingly quite normal ones, like castling kingside.

6. Qb3 is obviously devastating if Black were forced to capture on e4, which she isn't. The immediate tactical problem for White is that 6...d5! now can't be met with 7.e5, because the White queen would find herself badly placed after 7...dxc4 8.exf6.

The "forcing" 6. Bxf6 is a terrible, horrible, no good, very bad move. Apart from giving up the bishop pair, the Black queen that appears on f6 will target a1 (forcing the passive 7.Nd2) and also f2 in combination with a bishop on c5.

 6.e5  is the most popular and strongest move for White.