The Two Knights Defense (also called the Prussian Defense) is a chess opening that begins with the moves: 1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6
Black's third move threatens to capture White's pawn on e4. First recorded by Giulio Cesare Polerio in , White’s traditional and most direct approach is 4.Ng5, threatening Black’s f-pawn, defended only by the king. The theory of this opening was extensively developed in the 19th century, when Mikhail Chigorin demonstrated Black's compensation in the line 4.Ng5 d5 5.exd5 Na5 6.Bb5+ c6 7.dxc6 bxc6, where Black gambits a pawn for a lead in . White also frequently plays 4.d3, a more continuation often transposing to the Giuoco Pianissimo, or 4.d4, usually transposing to the Scotch Gambit.
The Two Knights is a more aggressive continuation than the Giuoco Piano (3...Bc5), which avoids the 4.Ng5 line. David Bronstein argued that the term "defense" doesn't capture the opening's true character and suggested "Chigorin Counterattack" as a more appropriate name. The Two Knights Defense has been played and analyzed by many aggressive players, including Chigorin, Paul Keres, and world champions Mikhail Tal and Boris Spassky.
Chigorin's line: 4.Ng5 d5 5.exd5 Na5 6.Bb5+ c6 7.dxc6 bxc6
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 , although Black gains compensation with quick and initiative. The standard reply is 5...Na5, which has been called the Chigorin Gambit, 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 analysis and play.
Old main line: 8.Be2 h6 9.Nf3 e4 10.Ne5
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 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.
10...Bd6
10...Bd6 is Black's most common next move. White's most common reply is 11.d4, though 11.f4 is an alternative. Black's most common reply after 11.d4 is 11...exd3 e.p., though 11...0-0 and 11...Qc7 are also seen, and 11...c5 has been advocated by Alexander Graf.
After 11.d4 exd3, the line continues 12.Nxd3 Qc7 (12...0-0 is also seen), after which White has a major choice on the thirteenth move. 13.b3, 13.h3, and 13.Nd2 are the primary options. 13.b3 is considered the main line, exerting control over c4 and enabling a queenside fianchetto. Play typically continues 13...0-0 14.Bb2, followed by 14...Ne4, 14...Re8, 14...Nd5, or 14...Bf5.
11.f4 is not regarded as not posing Black as many problems after 11...exf3 12.Nxf3 0-0. White may continue with 13.d4 or 13.0-0.
10...Bc5
10...Bc5 is Black's main alternative move, and prevents an immediate 11.d4. White's typical move is instead either 11.c3, preparing 12.b4 or 12.d4, or 11.0-0.
After 11.c3, Black has a choice between 11...Bd6, 11...Qc7, and 11...0-0. After 11...Qc7 12.d4, the line 12...exd3 13.Nxd3 Bd6 seems to give Black excellent compensation. 11...Bd6 may continue 12.f4 or 12.d4.
After 11.0-0, possible continuations include 11...Qd6, 11...Qd4, and 11...0-0. The 11...Qd6 line often leads to a series of exchanges with 12.d4 Qxd4 13.Qxd4 Bxd4 14.Nc4 Nxc4 15.Bxc4.
Other lines
10...Qc7, played by Carl Göring, often transposes to lines with 10...Bd6 or 10...Bc5. White's main move is 11.d4; 11.Ng4 is possible but regarded as inferior. 10...Qd4 tends to continue with 11.f4 Bc5 and then 12.Rf1, defending against the checkmate threat. White seems to retain an advantage but cannot castle kingside.
Steinitz Variation: 8.Be2 h6 9.Nh3
9.Nh3 is a variation popularized by Wilhelm Steinitz. Although it did not bring Steinitz success in his famous 1891 cable match against Mikhail Chigorin, Bobby Fischer revived it in the 1960s. Nigel Short led a second revival of 9.Nh3 in the 1990s, and today it is thought to be about equal in strength to the more common 9.Nf3.
9.Nh3 became slightly more frequently played than 9.Nf3 in the 21st century, but it has still lost popularity overall due to the rise of 8.Bd3. Black's common replies include 9...Bd6, 9...Bc5, 9...g5, 9...Be7, and 9...Bxh3. Black is regarded as having an easier-to-play game after 9...g5 and 9...Be7, but 9...Bd6 and 9...Bc5 are more frequent.
Bogoljubow Variation: 8.Qf3
Named after Efim Bogoljubow, 8.Qf3 pins Black's pawn on c6 to the rook on a8. It is possible to sacrifice the rook with 8...cxb5, a line played by Joseph Henry Blackburne, though more common are 8...Be7 (developing) and 8...Rb8 (unpinning). Another option is 8...h6, recommended by Jan Pinski.
After 8...cxb5 9.Qxa8, a possible continuation is 9...Qc7 10.Qf3 Nc6 11.c3 Bg4 12.Qg3 Nd4 13.Na3 Ne2.
New main line: 8.Bd3
8.Bd3 was still considered a mistake by Jan Pinski in 2004 but is now the main line of the 4.Ng5 variation. The line has some continuations involving White sacrificing a knight for two more pawns, leaving material equal given the pawn captured on the fifth move.
Deviations from Chigorin's line
6...Bd7
After 6...Bd7, White usually responds with 7.Qe2, defending the bishop and threatening Black's pawn. Black may reply with either 7...Bd6 or 7...Be7. Tim Harding wrote that 6...Bd7 was known to Polerio, who lived when 4.Ng5 d5 5.exd5 Nxd5?! was common.
Morphy Variation: 6.d3
Instead of retreating the bishop, White may instead play to hold the gambit pawn with this move. Paul Morphy preferred this line, which has been named after Lionel Kieseritzky and is also known as the Morphy Variation. It is relatively unpopular since Black obtains good chances for the pawn with 6...h6 7.Nf3 e4 8.Qe2 Nxc4 9.dxc4 Bc5 and White effectively concedes the bishop pair. David Bronstein once tried the piece sacrifice 8.dxe4!? with success, but its soundness is doubtful.
Fritz Variation: 5...Nd4
German master Alexander Fritz (1857–1932) suggested 5...Nd4 to Carl Schlechter, who published analysis of the move in Deutsche Schachzeitung in 1904, concluding that it is fully viable.Carl Schlechter, Zum Zweispringerspiel im Nachzuge, Deutsche Schachzeitung, August 1904 pp 225-226, September 1904 pp 257-258, October 1904 pp 289-290 In 1907 P. S. Leonhardt disputed this assessment in his column for the Swedish journal Tidskrift för Schack, in which he advocated the now standard move (6.c3 b5) 7.Bf1!Paul Saladin Leonhardt, Teoretiska notiser - Preussiskt parti, Tidskrift för Schack, March 1907, pp 61-62
6.d6 now is inferior. After 6...Qxd6 7.Nxf7? Qc6! simultaneously attacks the pawn on g2 and the bishop on c4, and if 8.Nxh8? (relatively best is 8.0-0, giving up the bishop) 8...Qxg2 9.Rf1 Qe4+ 10.Be2 Nf3#. White should instead play 7.Bxf7+ Ke7 8.Bb3 Nxb3 9.axb3 h6 10.Nf3 e4 11.Ng1 Kf7 and Black had good compensation for the pawn in Bogolyubov-Rubinstein, Stockholm 1919.
Instead, White's best reply is 6.c3, after which the game tends to continue 6...b5 7.Bf1! Nxd5, followed by either 8.cxd4, 8.Ne4, or 8.h4. Black also has the option of playing 6...Nf5 or 6...h6 instead of the usual 6...b5.
6.c3 b5 7.Bf1 Nxd5 8.cxd4
After the usual 8...Qxg5 9.Bxb5+ Kd8, this line tends to continue with either 10.0-0 or 10.Qf3.
10.0-0 is typically followed by 10...Bb7 11.Qf3 Rb8 12.dxe5 (not 12.Qxf7 Nf6, which defends against mate on d7 while threatening mate on g2 and 13...Bd5, trapping White's queen) 12...Ne3 13.Qh3 Qxg2+ 14.Qxg2 Nxg2 15.d4, where White is a pawn up in a sharp position.
10.Qf3 may be followed by 10...Bb7 11.0-0, transposing to the above line, but 10...exd4 is also possible, as in Wei Yi vs. Jan-Krzysztof Duda, 2014 World Junior Chess Championship
6.c3 b5 7.Bf1 Nxd5 8.Ne4
This often continues with 8...Qh4, the Berliner Variation, named after World Correspondence Chess Champion Hans Berliner. The line was played in the famous game Estrin–Berliner, World Correspondence Championship 1965–68, eventually won by Black; this win was pivotal to Berliner's eventual championship victory. That game, which saw Black embark on a very sharp sacrificial path, continued 9.Ng3 Bg4 10.f3 e4 11.cxd4 Bd6 12.Bxb5+ Kd8 13.0-0 exf3. In 1971, IM / GMC Estrin later published a suggestion of the move 14.Qb3!? as an improvement on the game continuation, and this possibility has continued to interest many players.
8...Ne6 is the main alternative to 8...Qh4. It is a quieter line typically continuing with 9.Bxb5+ Bd7 10.Bxd7 Qxd7.
Ulvestad Variation: 5...b5
This line is related to the Fritz Variation as they share a common subvariation. American master Olav Ulvestad introduced 5...b5 in a 1941 article in Chess Review. White has only one good reply, the counterintuitive 6.Bf1!, protecting the undefended pawn on g2, so White can answer 6...Qxd5 with 7.Nc3. Both replies 6.Bxb5 Qxd5 7.Bxc6+ Qxc6 and 6.dxc6 bxc4 7.Nc3 are weak for White. After 6.Bf1, Ulvestad's first idea 6...h6 is refuted by 7.Nxf7. The move 6...Nxd5, Ulvestad's second idea, is considered Black's best response. Black can also transpose to the Fritz Variation with 6...Nd4, making another advantage of 6.Bf1 apparent; the bishop is not attacked as it would be if White had played 6.Be2.
5...Nxd5?!
This recapture is extremely risky. Albert Pinkus tried to bolster this move with analysis in 1943 and 1944 issues of Chess Review, but White gets a strong attack with either the Lolli Attack (6.d4), which Bobby Fischer thought to be very strong, Fischer's analysis of this line is in game 45, Fischer–Bisguier, 1963 or the sacrificial Fried Liver Attack (6.Nxf7), which leaves Black's king in the middle of the board after 6...Kxf7 7.Qf3+ Ke6 8.Nc3. These variations are usually considered too difficult for Black to defend .
Traxler Variation: 4...Bc5