Chess Opening Theory/1. e4/1...Nf6/2. Bc4

White leaves the e4-pawn undefended. Though they can recover the pawn tactically with Bxf2+ this riposte from White is not as dangerous as it looks and Black ends up with a superior position.

2...Nxe4 takes the hanging pawn. To recover the material, White has a flashy manoeuvre where they sacrifice the bishop and trade for Black's knight. 3. Bxf7+ Kxf7 4. Qh5+ (4. Qf3+?? Nf6 -+}) Kg8 5. Qd5+ e6 6. Qxe4. However, White has given up their space advantage, while Black's king is quite safe and they have easy development. 6...d5 7. Qe2 e5! (sacrificing the e-pawn to open their bishop and develop with tempo) 8. Qxe5 Nc6. Black far ahead in development.

There is therefore no reason why Black should wish to avoid taking the pawn, but alternatives include 2...d5!? 3. exd5 Nxd5 (3. Bxd5? Nxd5 4. exd5 Qxd5 and watch out for Black's attack on g2). 2...e5!? transposes into the bishop's opening.

Josef Krejcik (1885―1957) was an Austrian chess master who originated this line in 1922.^[1] A poor defence by Black led to their resignation on turn seven (see sample game).

Josef Krejcik v. Adolf Gottlieb, 1922.
Landstrasser Schachbund, Vienna.^[1]

1. e4 Nf6 2. Bc4

The Krejcik variation, an unconventional continuation of the Alekhine defence.

2...d6!?

Better was simply to take the pawn, 2...Nxe4. 3. Bxf7+ Kxf7 4. Qh5+ Kg8 5. Qd5+ e6 6. Qxe4 d5 7. Qe2 e5! Qxe5 Nc6 ∓. Perhaps Black is wary of White's unusual continuation and contemplates transposing into a solid Pirc defence for safety.

3. Nc3 Nxe4?

Only now, having given White the opportunity to bring out another defender of the pawn, does Black take the pawn. If 4. Nxe4 d5! 5. Bd3 dxe4 6. Bxe4, material is even and Black has the initiative.

4. Bxf7+! Kxf7 5. Nxe4

Black loses castling rights, but the position is close to even.

5...e5 6. Qf3+ Kg8??

6...Ke8 and Black is still in the game.

7. Ng5 1-0

Threatens 8. Qf7#. If 7...Qxg5 8. Qd5+! Be6 9. Qxe6#. The best defence is to give up a bishop: 7...Be6 8. Nxe6 Qe7. Black resigns.

1. e4 Nf6 2. Bc4

Wikipedia has related information at Alekhine's Defence.