Defending Against 1.e4 e5 2.Nf3 2.Nc6 3.Bc4.
The most natural response to 1.e4 is 1...e5. After the equally natural moves 2.Nf3 Nc6, White has the choice of several systems. One of the sharpest is 3.Bc4 which immediately targets f7 which is Black's weakest square.
The Two Knights Defense
Based on my experience the most popular response among high school players is 3...Nf6 which is known as the Two Knights Defense. The most popular move for White is then 4.Ng5. This violates the basic opening principle (or maybe just rule of thumb) which says that no piece should be moved twice until every piece is moved once. In this case however, the violation is justified by the fact that Black has no easy way to defend f7 against the dual threat of the knight and the bishop.
Black can if he wishes ignore the threat with 4...Bc5 which is known as the Traxler Gambit. This is very exciting, but completely sound. The recommended move is 4...d5, which blocks the bishop's attack on f7. However, Black faces another choice after 5 exd5.
Practice has shown that Black's best move here is the odd looking 5...Na5. This move also moves a piece twice when other pieces haven't moved once. On top of that, it places a knight on the edge of the board.
While the Two Knights with 5...Na5 is considered a perfectly sound approach, it is tough to play without a decent level of book knowledge It seems to me that it is an openings where the common opening rules of thumb (like not placing a knight on the edge of the board) get violated more often than usual. One main continuation goes 7.Bb5+ c6 8.dxc6 bxc8 9.Qf3.
These are not the easiest moves to find over the board in a sixty minute game if you haven't seen them before.
The Fried Liver Attack.
After the more natural looking 5...Nxd5, White has the option of playing the Fried Liver Attack where White sacrifices a knight for a pawn with 6.Nxf7 in order to expose the Black king and draw it out to the middle of the board. Apparently the name derives from the fact that Black frequently winds up as dead as a piece of liver.
The Fried Liver Attack is not necessarily winning for White but it is not easy for Black to defend after 6...Kxf7 7.Qf3+ when Black is forced to play 7...Ke6 in order to defend the knight on d5.
One possible continuation is 8. Nc3 Ncb4 9. Qe4 c6 10. a3 Na6 11. d4 Nac7. However, 6.d4, delaying the knight sacrifice may actually be a better move for White. This allows White to add his other bishop to the attack quickly and it turns out that Black doesn't have anything that particularly improves his defensive chances. For example 6...Be7 7.Nxg7 Kxg7 8.Qf3+ gives White a revved up Fried Liver.
The Italian Game
In the ten years that I have been coaching at Prospect, I may have seen the Fried Liver a dozen times in matches. To the best of my recollection, Black won most if not all of them. So what is Black to do if he doesn't want to allow the Fried Liver and he doesn't want to have to find a lot of counter-intuitive moves after 5...Na5? I've seen some players go for 3...h6 to prevent 4.Ng5, but neglecting development isn't a good idea. My preference is simply 3...Bc5 leading to the Giuoco Piano aka the Italian Game.
Now 4.Ng5?? simply loses the knight to 4...Qxg5. If White plays 4.0-0, after 4...Nf6, Black can meet 5.Ng5 with 5...0-0.
3...Bc5 doesn't eliminate the possibility that White may sacrifice material to get a nasty attack. 4.b4 is the Evans Gambit which can be very tricky to handle. However, White often plays more quietly with 4.0-0 or 4.d3 and even if he chooses a more aggressive line, overall I think opening principles are violated much less frequently than in the Two Knights.
To sum up: After 1.e4 e5 2.Nf3 Nc6 3. Bc4
- The Two Knights Defense 3...Nf6 is perfectly sound however after White's most common move, 4.Ng5, Black is likely to wind up in tricky positions that are hard to play without some prior study.
- 3...Bc5 is also perfectly sound and the odds of Black reaching a position that requires less book knowledge is much greater.