The Blackmar-Diemer Gambit is a very controversial gambit starting with 1.d4 d5 2.e4 dxe4 3.Nc3 intending 3...Nf6 4.f3. It can also be used against the Scandinavian Defence: 1.e4 d5 2.d4!?.
Blackmar's original idea, 3.f3, is ineffective because of 3...e5, so Diemer improved upon the idea by interposing the knight moves 3.Nc3 Nf6, so that if 4.f3 e5?!, 5.dxe5 hits Black's knight on f6.
Black can still play 3...e5 against 3.Nc3, but then I quite like White's chances after 4.Nxe4, which is a significant advantage of having played 3.Nc3 instead of 3.f3.
The main accepted line of the gambit arises after 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Nxf3. White has an extra developing move and a half-open f-file in return for the sacrificed pawn.
I believe that the Blackmar-Diemer is very dangerous, but of marginal soundness - Black can probably get a theoretical edge against it with precise defence. However, it has what Tim McGrew called a high "caltrop coefficient": White can get some very fine attacking wins if Black goes wrong.
The most popular way for White to meet this is the Studier Attack with Bc4, 0-0, Qe1, Qh4 and Bh6. The typical attacking position is illustrated in the diagram on the left. White aims to play Bh6 and exchange off the bishop on g7, and then follow up with Ng5, Rxf6 (making good use of the f-file) and Qxh7#.
Black will want to disrupt this attacking plan in some way. In this position, the most reliable response is to play ...Bg4, with the threat of ...Bxf3 followed by ...Qxd4+. After White attends to that threat, Black can aim for ...e7-e5 or ...Qd8-d7-g4.
This diagram illustrates the kind of attacking position that White wants. White is threatening to play Nxd5 or Nce4, threatening to exchange off the knight on f6 and play Qxh7 mate. In the game, Black tried 16...c6, but after 17.Nce4!, White still had a winning attack.
The Studier Attack is very dangerous, but personally I have never been that big a fan, as it's rather inflexible and predictable, and White has been having difficulties getting adequate compensation for the pawn against the aforementioned ...Bg4xf3 idea.
My main recommendation for White against the ...g6 setups is instead to play Bf4, Qd2 and 0-0-0. If Black decides to leave the king in the centre, then White can get good play using the half-open e and f-files, and by playing Bh6 at a good moment, leaving Black's kingside bare.
Thus, Black's most challenging reply to the "Long Bogo" is to play ...Bg7 and ...0-0 immediately (which in practice Black usually does), leading to the position on the left, and to aim for queenside counterplay against the centre and the white king with ...c5, ...a6 and ...b5-b4.
In many lines White can use the standard h-pawn hack with h2-h4-h5, and if Black captures on h5 with a piece, a Rxh5 exchange sacrifice is sometimes strong. However, as White is missing an f-pawn, White should generally play Bh6 before playing h4, because if Black can meet h4 with ...h5, it is then hard to engineer g2-g4, and this takes a lot of the sting out of White's attack. If Black plays ...Bg4 or ...Bf5, then White can refine this plan and play h3 and g4, and then aim for h3-h4-h5.
The diagram on the right is a good illustration of the kind of attacking position that White is after.
After 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Nxf3, Black can play 5...Bf5 immediately. I believe that this is one of the most critical replies. When Black puts the bishop on this square, a standard plan is to kick the bishop back to g6 by playing g2-g4, and then follow up with Nf3-e5 and h2-h4.
In the line with 5...Bf5, White isn't ready to play 6.g4 at once, so White needs to start with 6.Ne5, intending 7.g4. Most often, play will reach the diagram on the left. White will follow up with Bg2 or Qf3 to disrupt Black's ideas of ...Bb4 followed by ...Be4, and then go for h2-h4. It is generally a good idea to wait until Black plays ...h6 or ...Nbd7 before playing Nxg6.
This position shows the kind of attack that White is looking to build up in the 5...Bf5 6.Ne5 line and similar lines where White kicks the black light squared bishop back to g6. Here White has tempted Black into grabbing the pawn on c2, leaving the bishop rather offside on a4, while White threatens to storm the black king with the kingside pawns (despite having also castled to the kingside!).
I think White generally has good compensation for the pawn in these lines. The main problem with 6.Ne5, although this response is rarely encountered in practice as it looks rather odd, is that 6...c6!? 7.g4 Be6! is hard to crack.
The main alternative, which has been examined by Stefan Bücker, is to offer to exchange off the light squared bishops with 6.Bd3. White argues that this trade helps White's development, bringing White a move closer to castling queenside, and removes a useful defensive piece.
It's still debatable if White's lead in development and attacking chances are worth a pawn, but in the diagram position on the left, White has ideas of engineering the disruptive d4-d5 pawn push, and playing Ne5 or Ng5.
The diagram position on the right reflects the kind of chaos that White can hope to generate in this variation.
Black has started a queenside attack, but in the meantime White is attacking through the centre and has the ongoing threat of g4-g5 and ideas of Nh4-f5. The position is rather difficult to assess.
However, practical examples with this 6.Bd3 line have been sparse, as most players have stuck with Diemer's preference 6.Ne5.
Black argues that by playing 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Nxf3 c6, Black can wait for White to commit the bishop on f1 before playing ...Bf5. If 6.Ne5 Nbd7, 6.Bc4 Bf5, or 6.Bd3 Bg4.
It's worth noting that Black can avoid the 6.Bd3 Bg4 line by playing 4...c6 first, whereupon White's best continuation is 5.Bc4, and then 5...exf3 6.Nxf3 Bf5 transposes to the 6.Bc4 Bf5 line, so 6.Bc4 Bf5 is by far the most important continuation.
Then White most often plays 7.0-0 e6 8.Ne5, but then Black plays 8...Bg6! (diagram). White can refine this idea with 8.Ng5!?, with ideas of sacrificing on e6 or f7, but again after 8...Bg6!, although Black has to defend more accurately here, White still doesn't have enough compensation for a pawn.
Thus, after 5...c6 6.Bc4 Bf5, White should probably instead try 7.Bg5, as recommended by Stefan Bücker. Again, the idea is generally to castle queenside, and to tie Black down on the kingside, while threatening the d4-d5 push in various positions.
Then if Black plays something like 7...Nbd7, White continues with 8.Qe2 and 9.0-0-0. If 7...e6, however, 8.Qe2 can be met by 8...Bb4!, which stops 9.0-0-0, as then 9...Bxc3 10.bxc3 would leave White's king exposed. But as 7...e6 stops the bishop on f5 from retreating to e6 or d7, 8.Nh4!? now forces the exchange of this bishop: 8...Bg6 9.Nxg6 hxg6 10.Qd3 (diagram), an idea that comes from Lev Gutman.
In this position Black has to beware of possible sacrifices on e6, and White will continue with 0-0-0, h4 and Rhf1, tying Black down to the defence of f7.
The most popular defence to the Blackmar-Diemer Gambit is 5...Bg4, pinning the knight on f3, and keeping open the possibility of swapping it off at the right moment. I don't think it's one of the most critical, though. After 6.h3!, 6...Bh5 7.g4! Bg6 8.Ne5 tends to transpose into the variations with 5...Bf5 6.Ne5 e6 7.g4 Bg6, but denying Black the critical alternative 6...c6 7.g4 Be6. I think White has enough for the pawn here.
You might wonder, why play 6.h3, isn't it just a waste of a tempo to provoke Black into exchanging? Well, not quite, because if White just develops normally, Black is more likely to just leave the bishop on g4, keeping the knight on f3 in an awkward pin. Also, the move 6.h3 prepares g2-g4, which threatens to kick the black knight away from f6 at a good moment with g4-g5. Then in various lines the bishop on f1 can come out to g2, rather than its more traditional post on c4.
More consistent is to meet 6.h3 with 6...Bxf3 7.Qxf3 c6 (7...Nc6 walks into an awkward pin after 8.Bb5, threatening 9.d5).
Then it's not clear if White can get enough compensation for the pawn with the routine 8.Be3, but White has a more violent alternative in 8.g4!?, sacrificing a second pawn on d4, with the idea of quickly bringing a rook to f1, playing g5 and then breaking through to Black's weak point on f7 (this highlights another useful point behind 6.h3).
If Black takes on d4, White can play Be3 and 0-0-0 and gets very dangerous compensation, while if Black doesn't take, White tends to get into favourable versions of the 8.Be3 lines.
5...e6 is a solid but passive defence to the Blackmar-Diemer, shutting in the bishop on c8, and aiming for rapid kingside development. If White plays 6.Bd3, 6...c5! is quite awkward to meet, so White does best to pin the knight on f6 with 6.Bg5. Then if 6...c5, 7.Bxf6, intending 7...Qxf6 8.Bb5+ followed by 0-0 and/or Ne5, gives White a dangerous initiative. Thus, Black tends to continue with 6...Be7 (diagram).
Here White has two main options.
7.Bd3 is the traditional main line. Joe Gallagher and James Rizzitano have recommended 7...c5, but I think White can get away with 8.dxc5, followed by either 9.0-0, or 9.Qe2 and 10.0-0-0, with plenty of compensation for the pawn in either case. Also, Black should probably avoid the "automatic" 7...0-0, because then 8.0-0 followed by Qe1 and Qh4 gives White a dangerous attack, with the bishop on d3 pointing at h7, and the threat of Rxf6 sacrifices.
The most challenging reply to 7.Bd3 is rather 7...Nc6, with the idea of exchanging off the bishop on d3 with ...Nb4 and ...Nxd3.
White can continue with either 8.a3 (preventing ...Nb4, but at the cost of time) or 8.Qd2 (allowing the exchange). Lev Zilbermints has favoured 8.0-0 Nxd4 9.Kh1, sacrificing a second pawn, but if Black isn't tempted by 9...Nxf3?! 10.Qxf3, which is dangerous for Black, Black has good chances of getting the upper hand.
The other way to continue is 7.Qd2, preparing queenside castling. Here White can continue with Qd2-f4-h4, and get the queen across that way. White has had many nice wins where Black has played ...0-0 and ...h6, and fallen for a very strong Bxh6 sacrifice.
I find it interesting to compare the Blackmar-Diemer Gambit with the main line of the 3...Qd8 Scandinavian. After 1.e4 d5 2.exd5 Qxd5 3.Nc3 Qd8 4.d4 Nf6 5.Nf3, we reach the position in the diagram on the left. Yes, it's a Blackmar-Diemer Gambit but with an extra pawn on f2!
However, there are certainly some lines where that pawn on f2 gets in the way - it reduces White's scope for ganging up on f7, and it stops White from playing Qe1 followed by Qh4 or Qg3 in various lines.
In the 3...Qd8 Scandinavian White thus often plays more positionally than is normal in the Blackmar-Diemer, e.g. 5...g6 6.Bc4 Bg7 7.0-0 0-0 8.a4, or 5...Bg4 6.h3 Bxf3 7.Qxf3 c6 8.Be3 followed by normal development. That said, as I like to mix things, if you gave me that extra pawn on f2, I'd still prefer Bf4, Qd2 and 0-0-0 in the first line, and 8.g4 (sacrificing the d4-pawn) in the second line.
So, let's get on to looking at the accepted lines of the gambit, starting with 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3, and now 5.Nxf3 or 5.Qxf3.
In the next chapter, I'll look at the 3rd and 4th move alternatives for both sides. Of these, the Vienna Defence (4...Bf5), as recommended by David Smerdon in Smerdon's Scandinavian, is as critical a test of White's gambit as taking on f3 and then following up with ...Bf5.