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I'm new to chess. In the position below, lichess analysis says that Nxd5 is the best move for white. But can't black just capture the knight afterwards with his pawn and threaten white's pawn and rook in the top right hand corner after white's knight is dead? What makes Nxd5 the best move here?

 [FEN "r3k3/p5R1/p1p4r/2bbp2p/8/P1N3KN/1P5P/R7 w q - 0 1"]
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  • Without the bishop on d5 what threats does white have? How can he/she force Black to make moves that leave black rather tied down? And BTW in English the Tower is called a "Rook" - English being as logical as it it usually is. – Ian Bush Dec 7 '19 at 21:42
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    Also can Black still castle Queen Side? (Hint!) – Ian Bush Dec 7 '19 at 21:45
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First, you have to understand that computers sometimes play non-human moves because they see things that humans most times do not. It probably sees threats to the white king based on the black bishop pair.

In this position, white is just lost, and those bishops on d5 and c5 working together are incredibly strong, so trading off one of them is desirable. What makes it a little strange is that it gives black connected passed pawns, but they are hard to maintain, let alone advance, due to the active white rooks, and unfortunate black Ra8, but it still does not change the outcome.

Also, the Ra8 is not lost to Nd5 cd; Rg8 Bf8; Rf1 Ke7, but it does tie black up some, which is another reason that Nd5 was best. It is hard to finish his development.

 [FEN "r3k3/p5R1/p1p4r/2bbp2p/8/P1N3KN/1P5P/R7 w q - 0 1"]

 1. Nxd5 cxd5 2. Rg8+ Bf8 3. Rf1 Ke7 4. Rf5 {with a little counterplay that requires accuracy from black} Rf6! 5. Rxh5 (5. Rxe5? Kf7 6. Rh8 Bd6 7. Rxa8 Bxe5+ 8. Kg2 Bxb2 9. Rxa7 Kg6-+)  Rc8! 6. Rxe5 Kd7!! 7. Rd5 Bd6+ 8. Rxd6+ Rxd6 9. Rg7 Kc6 10. Rxa7 Kb6 {with an easy win from here}
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    I would have made that move. And I am human. The two bishops are too powerful to allow to remain. – yobamamama Dec 10 '19 at 23:43
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(Disclaimer: this is basically a (hopefully) developed form of yobamamama's answer with some opinions of my own thorwn in. The latter is the reason why I decided to write this up an independent answer. Feel free to edit or combine with yobamamama's answer)

First, I think the move is reported as best not for its merit; but it might be actually forced. Black is threatening 1.__ h4+ 2.Kg4 Be6+ 3.Kg5 (3.Kf3 Bxh3) 4.Be3+. To stop this threat, other than 1.Nxd5, white has following options (may not be exhaustive)
a) 1.Ng1 (this retreat looks like a very reasonable move here)
b) 1.Re1 (this look a kicking back move; may not work)
c) 1.Ng5 (risky)
[The move 1.Rf1 is not good enough. Black can play 1.__ h4+ 2.Kg4 Be6+ 3.Kg5 Be3+ 4.~f4 Rh8]
[I think 1.Ng5 is risky due to 1.__ Kf8; i am not sure about this]

Coming back to the move 1.Nxd5 cxd5. Now, 2.Rf1 seems to be superior to outright 2.Rg8+ suggested by Phishmaster(at least when you play against a human player). Note that 2.Rf1 threatens 3.Rg8+. This ties up black's options for a practical game (still, options 3.Rh8 and 3.Rd8 look good for black).

 [FEN "r3k3/p5R1/p1p4r/2bbp2p/8/P1N3KN/1P5P/R7 w q - 0 1"]

1. Nxd5 cxd5 2. Rf1
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  • I commented on this very position, Rf1 is not superior to Rg8. It is an easy and SIMPLE loss after just 0-0-0, and I went into detail in my back-and-forth with yobamamama. The problem is that doubling on the 7th here brings has no threat, and does nothing to contain the passed pawns. The move Rg8, while still losing, at least is serious counterplay, requiring precision by black. In other words, Rf1 easy win, Rg8 not so easy. – PhishMaster Dec 8 '19 at 21:26
  • Why are you so sure black can castle? Nobody mentioned that the black king never moved! – Cyriac Antony Dec 9 '19 at 8:21
  • What the difference between best on its merits, versus being forced? – Acccumulation Jan 17 at 2:41
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After the recapture white moves his rook to f1 and then next move to the seventh rank (f7).

 [FEN "r3k3/p5R1/p1p4r/2bbp2p/8/P1N3KN/1P5P/R7 w q - 0 1"]

1. Nxd5 cxd5 2. Rf1 null 3. Rff7

Note that by 2.Rf1, white threatens 3.Rg8+. Also, this brings the other rook into play. The game may be still lost, but this probably gives better fighting chances.

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  • -1. The 7th is easily covered there after Rf1? by just 0-0-0 Rff7 and then any number of moves defends. The computer likes Rc6, but the even the natural Bb6 evals at -3.75. Then the pawns roll, and are incredibly dangerous. – PhishMaster Dec 7 '19 at 23:13
  • @PhishMaster - I did not use a computer. But surmised what would happen after nxb .Else why give black connected passed pawns. – yobamamama Dec 8 '19 at 3:04
  • Read my comment above. I explained it all in detail using a computer and 30+ years of experience as a Master, and you can play it out on the PGN board I inserted. – PhishMaster Dec 8 '19 at 3:05
  • @PhishMaster -- Whatever. White is in a bad way. What does white do after pxn? The two bishops are very strong. But the connected pawns now makes up for losing one of them. White should lose no matter what. – yobamamama Dec 8 '19 at 4:27
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    @Rewan, that is great too. I like full be careful. – PhishMaster Dec 11 '19 at 14:44

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