Two Knights Defense

The Two Knights Defense (also called the Prussian Defense) is a chess opening that begins with the moves:

First recorded by Giulio Cesare Polerio (c. 1550 – c. 1610) in c. 1580, the theory of this opening was extensively developed in the 19th century. Black's third move, attacking White's pawn on e4, is a more aggressive continuation than the Giuoco Piano with 3...Bc5. White’s most direct approach is 4.Ng5, immediately attacking Black’s vulnerable f7-pawn. David Bronstein noted that the term defense doesn’t capture the opening’s true character and suggested Chigorin Counterattack as a more accurate name. The Two Knights Defense has been played and analyzed by many aggressive players including Mikhail Chigorin, Paul Keres, and world champions Mikhail Tal and Boris Spassky.

The move 4.Ng5 attacks Black's pawn on f7 in combination with White's bishop on c4. Common during the era of Romantic chess, the move attracted criticism from players of the Classical school who argued that it violates opening principles because it moves a piece twice and attacks before development is complete. The leading German master of the early 20th century, Siegbert Tarrasch, famously called 4.Ng5 "a duffer's move" (ein richtiger Stümperzug), also translated as "a typical example of a bungling move". 4.Ng5 experienced a revival in popularity in the late 20th century after being adopted in the 1990s by Alexander Morozevich and Nigel Short, though it remains less common than 4.d3. It has been employed by world champions Wilhelm Steinitz, Bobby Fischer, Anatoly Karpov, Garry Kasparov, and Viswanathan Anand.

The only good defense for Black is 4...d5, blocking White's bishop's line of attack. After 4...d5, 5.exd5 practically wins a pawn by force, although Black gains compensation with quick development and initiative. The standard reply is 5...Na5, attacking the bishop before White can move the pawn and resume the attack. The main line continues 6.Bb5+ c6 7.dxc6 bxc6, after which 8.Bd3, 8.Be2, and 8.Qf3 (Bogoljubow Variation) are all common continuations. 8.Ba4? is a notable mistake; Black can win material after 8...h6 9.Nf3 (or 9.Nh3 Qd4) e4, as White's knight must either retreat to g1 or move to e5, in which case 10...Qd4 wins material. 6...Bd7 is also possible, as is 6.d3 (Kieseritzky Variation).

Notable alternatives to 5...Na5 are 5...Nd4 (Fritz Variation) and 5...b5 (Ulvestad Variation), which usually transpose into each other. Simply recapturing with 5...Nxd5?! is considered too risky of a move to play at the board, although it has not been definitively refuted.

Black can also ignore the threat to f7 on the fourth move, most notably with 4...Bc5!?, the Traxler Counterattack, which leads to wild complications. In general, the 4.Ng5 variation is known for extensive theoretical analysis and sharp tactical play.

8.Be2 is usually followed by 8...h6, where 9.Nf3 almost always continues 9...e4 10.Ne5. This leads to a position where White is behind in development but has an extra pawn as well as a better pawn structure; Black has two isolated pawns. This line was the main line of the 4.Ng5 variation since the 19th century, when Mikhail Chigorin demonstrated Black's compensation, but has been surpassed in popularity by 8.Bd3 and rivaled by 9.Nh3 and 8.Qf3.

After 10.Ne5, Black has four main moves: 10...Bd6, 10...Bc5, 10...Qc7, and 10...Qd4.