Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e5/3. dxe5/3...Ne4/4. Qc2

This line is sometimes called the Steiner variation. White immediately attacks the Ne4, but takes care not to put the queen on a square where Black could attack it while developing (as would be the case after 4.Qd3 or 4.Qd4). Now any retreat by the Ne4 would mean that Black loses their advance in development, in which case they no longer has any compensation for the sacrificed pawn. Thus Black must continue to develop while trying to keep the Ne4 on its square, but that is by no means easy. Borik thinks 4.Qc2 is the one "that gives Black the most problems to solve", but Lalic does not agree at all, stating that the reply "4...Bb4+ [....] followed by d7–d5 ensures Black rapid development and plenty of counterplay, it is for this reason that 4.Qc2 is not on the danger list".

4...d5
1.d4 Nf6 2.c4 e5 3.dxe5 Ne4 4.Qc2 d5

The reply 4...d5 protects the Ne4, opens the way for the Bc8 and leads to a good position for Black after various natural White's moves:
 * 5.cxd5 allows Black to develop its queen with 5...Qxd5, when White is not able to keep the e5-pawn and parry the various threats on the a5–e1 diagonal on the same time, for example 6.Nc3 Nxc3 7.Qxc3 Nc6 with the double threat Qxe5 and Bb4, or 6.Nd2 Bb4 7.Ngf3 Nc6 8.a3 Bxd2+ 9.Bxd2 Nxd2 10.Qxd2 Qxd2+ 11.Kxd2 Bg4 followed by O-O-O and Rhe8. Lalic points out that Black can also go for 5...Bf5! 6.Qa4+ c6 7.d6 Qb6 8.e3 Nd7 with advantage to Black.
 * 5.Nf3? Bf5 (threatening 6...Ng3) 6.Qa4+ Nc6 7.Be3 Bb4+ 8.Nbd2 d4! and White has more and more problems to solve with g7–g5 to come.

It does not, however, stand up against the best white continuation, so that Lalic considers it to be "objectively not entirely sound" and "dubious". After White's best reply 5.exd6 removing the Ne4 from its outpost with 5...Nxd6 would mean the failure of Black's dynamic strategy, so Black continues to develop with 5...Bf5, in the same time creating the threat 6...Ng3. Now again White has opportunities to go astray: But White can benefit from the pin of the Ne4 (as the Bf5 is not protected) with the paradoxical 6.Nc3! when White keeps their queen under the threat of the Bf5 but develops their pieces and attacks the Ne4 once more. Now the e4-knight has only two discoveries that protect the Bf5, but 6...Ng3 fails to the tactical 7.Qa4+ Bd7 8.dxc7 Qxc7 9.Nb5! and White wins. Thus Black has only 6...Nxd6 to keep the initiative.
 * 6.dxc7 is too greedy, giving an important tempo and opening the d-file for the Ra8. Black continues with Qxc7, Nc6 and O-O-O, then depending on the circumstances they can create pressure on the d3-square with Nc5 and Nb4.
 * 6.Qa4+ and 6.Qb3 lose at least two tempi as the white queen will again be under attack when Black plays Ne4–c5. Black easily develops their pieces with Nc6, Bxd6 and an attack along the central columns.

Now a retreat with the white queen would give a development advantage to Black, so White uses the fact that their sixth move has given them enough control of the e4-square to play 7.e4!, preparing the development of the Bf1 and attacking the Bf5. Unfortunately for Black any reasonable defence like 7...Qe7 or 7...Bg6 would give White the time to catch up in development, and remain a pawn up. Thus Black's best option is the piece sacrifice 7...Nxe4 to grab a pawn and tempt White into a fire of tactical pressure. The acceptance of the sacrifice with 8.Nxe4 gives Black enough play for the piece, e.g. after 8...Bb4+ 9.Ke2 Nc6 10.Be3 Qe7 11.f3 O-O-O. White does best to give up an exchange and continue their development with 8.Bd3! when after the possible 8...Nxf2 9.Bxf5 Nxh1 10.Nf3 White has an enormous lead of development for their material investment. White's position is to be preferred. Borik proposes to revive the variation with 10...g6, but then Lalic argues that 10...g6? 11.Bg5! Be7 12.Rd1 leaves White on top.

4...Bb4+
1.d4 Nf6 2.c4 e5 3.dxe5 Ne4 4.Qc2 Bb4+

Instead of 4...d5, more cunning is 4...Bb4+ to pin the white pieces before deciding what to do with the Ne4. White cannot reply 5.Bd2 as they would lose the bishop pair and Black would easily regain the e5-pawn with Nc6/Qe7/O-O/Re8. After 5.Nd2 this knight is misplaced and blocks the Bc1, so Black can open the game with 5...d5 in favourable circumstances, when White can continue in several directions but Black gets a good game in all cases:
 * 6.cxd5 will probably transpose into the 4.Qc2 d5 5.cxd5 variation, which is not very good as Black develops their queen with tempo.
 * 6.e3 is a bit passive, e.g. 6...Bf5 7.Bd3 Qg5! 8.g3 Nd7 9.Ngf3 and here Borik recommends 9...Qh5 with a good attack, a possibility that is also endorsed by Tseitlin.
 * 6.Nf3 was tried in 1987 for the first time by Stohl, but after 6...Bf5 7.Qb3 Nc6 8.cxd5 Nc5 9.Qx4 b5 10.Qf4 Qxd5 11.Qxf5 Ne4! Tseitlin considers "Black's threats are difficult to meet". Note that Lalic considers 6...Bf5 to be an error on the basis of the continuation 7.Qb3! Bxd2+ but they fails to consider Tseitlin's reply 7...Nc6.
 * 6.exd6 is considered an error by Lalic but after 6...Bf5! they only looks at 7.Qa4+ and fails to consider Borik's recommendation 7.a3 Bxd2+ 8.Bxd2 Qxd6, when Black has enough compensation for the pawn with their active Ne4 and Bf5.

The other possibility for White is 5.Nc3 so that the Bc1 is not blocked. Black continues with 5...d5 when White should avoid the natural continuation 6.cxd5?! Qxd5 7.Bd2 Qxd2+ 8.Qxd2 Nxd2 9.Kxd2 Nc6 because they have no satisfactory way to save the e5-pawn (e.g. 10.Nf3?! Bg4 11.e3 O-O-O+ 12.Kc2 Bf5+ etc.) so that Black gets a favourable endgame. After the better 6.exd6 (instead of 6.cxd5?!) 6...Bf5 7.Bd2 Nxd6 8.e4 Bxc3 9.Bxc3 Bxe4 Black has regained their pawn but White has the bishop pair and possibilities of an attack on the kingside. Instead of 8.e4 White can also try 8.Qb3 Nc6 9.e3 Qe7 10.Nf3 O-O-O but Black has good counterplay for the pawn.

Theory table
1.d4 Nf6 2.c4 e5 3.dxe5 Ne4 4.Qc2