Abdel Irada: <<•>The road to Stalingrad<•>>Invaders of Russia, in Napoleon's time as in Hitler's, can appreciate White's dilemma: He has placed forces deep in enemy territory in pursuit of a fast tactical knockout, and then found that those forces require supplies and logistical support.
In this case, it is the knight on g5 that finds itself "an army march[ing] on its stomach." In the puzzle position, White leads by the exchange and a pawn, but a temporary advantage is no advantage (unless, as we've seen in a few puzzles, it can be converted into a permanent one before it disappears).
In any case, Black calls the "advantage" into question with the key move:
<<•> 24. ...Qxh4 >
The supply line has been cut. The knight is thrice attacked and once defended; it either flees or dies where it stands.
Let us first assume the knight decides to run away, that it may live to fight another day. Where will it go?
It's pretty obvious that it would *like* to attack the queen, but (a) 25. Nf3??, Qxf2# offers nothing.
What is not yet obvious — but is about to become so — is that any knight move opens two fatal lines: the g-file and the c1-h6 diagonal, and together they spell DOOM.
< (1) 25. Nh3 ... >
This is the "safest" retreat, overdefending f2 to free the queen from its defense. But it meets with a shocking reply.
< 25. ...Qxh3!
26. Bxh3, Nh2†
27. Ke1, Rg1†
28. Bf1, Rxf1# >
As it turns out, much the same happens on (b) 25. Nxf7, Qh1† 26. Bxh1, Nh2† 27. Ke1, Rg1#. Likewise on (c) 25. Ne4, Qh1†, etc.
No retreat, then. But what if White also says, "No surrender"?
< (2) 25. Qf4†?!, Ka8 >
Of course, this check does no good unless it is the prelude to game-changing counterattack, so logically White continues:
< 26. Rxc6? ... >
The hope, of course, is that Black will play 26. ...bxc6?? 27. Bxc6#.
< 26. ...Nh2† 
>
Black wins the queen by discovered attack.
Finally, what if White keeps the queen out of danger and offers the rook immediately in hope of a favorable transposition?
<<•> (3) 25. Rxc6!, Bxg5 
>
This is clearly White's best practical chance. Whether Black takes on g5 with bishop or queen, he has two minor pieces for a rook and pawn and retains a slight advantage; this is in preference to the uncertain complications ensuing on 25. ...bxc6.
However, after the capture of the expeditionary force on g5, White will have to withdraw his sortie from c6 as well, and then hunker down for the long march home, his dreams of conquest and hegemony broken with his supply lines.
sac: 19...Rxh4
c3 is White's best defence, when Black gets only small advantage of 2 knights versus rook.
