Something incredibly rare happened at the biggest chess tournament in US history

The 2015 Sinquefield Cup is about to wrap up in St. Louis. It now looks like Armenian Grandmaster Levon Aronian, who lost the trophy to current World Chess Champion Magnus Carlsen at the Sinquefield Cup in 2013, will take home home the crown.

(Aronian was unable to defeat Carlsen in their final 2013 game, ending any chances of a playoff.)

Aronian has a full point lead going into the round 9, the final round of classical chess. If he draws or wins his game, he wins the tournament, and the second leg of the new Chess Grand Tour, which started with Norway Chess in June and will conclude with the London Chess Classic in December.

Aronian is trailed by the unlikely duo of France's Maxime Vachier-Lagrave and Russia's Alexander Grischuk, a pair of players who did not enter the event with high expectations. Meanwhile, last year's runway winner, Fabiano Caruana, is at the bottom of the standings.

This is the strongest field ever assembled for a chess tournament, by ranking. So it's no surprise that play has been wild and woolly, with pretty much everyone having a shot at beating everyone else.

In round 6, something astonishing happened, however.

America's Hikaru Nakamura, a potentially challenger for Carlsen's WCC title, was facing off against fellow American Wesley So, a young player who has come on strong over the past year.

Nakamura had the black pieces, So the white. Playing white gives an immediate edge in chess - white gets to move first, so black is always playing catch-up, a "tempo" or move behind. But Nakamura has the talent to win and win dramatically with both colors.

That said, no one expected the beat-down he put on So.

The game started out in elegant, historical fashion. It opened with a "hypermodern" series of moves, something called the King's Indian Defense, in which black allows white to build up a "big" center with a bunch of pawns and pieces, planning to undermine this position later. The idea is that instead of fighting for the center at the beginning of the game, black will start the battle in the middle game, aiming to turn the tables on white and go on the offensive.

As Nakamura noted after the game, the KID can be quite deceptive for the computer-prepared white player, because the position for white looks better - until suddenly, disastrously, it isn't. As you can see from the screenshot above, the computer thinks So is almost a whole pawn (1 point) better at move 11.

So selected an well-known KID continuation, putting his bishop on the e3 square. The general evolution here is that white will attack on the queen's side of the board, while black goes for an attack on the king's side, as GM Daniel King outlined in his recap of the game.

But this is a bit weird, as both white and black have castled kingside. So black has a direct attack down the g- and h-files, while white must pivot his attack along the diagonals or black's back ranks, via the c-file. If white can make it work, the attack will be beautiful and otherworldly, with a checkmate threat emerging seemingly out of nowhere. But black has the far more direct line of assault. It's a war of maneuver, in white's case, versus a simple siege for black.

The critical moment in the game occurred when So played his rook to d1, allowing Nakamura to unleash hell, starting with the move Rxf3.

From here it's just bonkers chess and Nakamura launches piece after piece into the attack, overrunning and overwhelming the white position. Look at the position after So's rook d1 move, above. And now look at the final position. The white king has been driven all the way to g6!!! And it's checkmate on the board!

Checkmates almost never happen in elite chess. Top GMs can see it coming and will almost always resign. But the brilliance of Nakamura's attack likely convinced So that his fellow American deserved the mate. So he allowed it to happen. To make matters worse for So, there was a game-for-the-ages variation that Nakamura didn't select, a line that delivered checkmate with a stunning queen sacrifice.

As it turns out, as brilliant as Nakamura's win was, it was only one of several gorgeous games at the Sinquefield Cup. Aronian also won a brilliant game, also against So, also with the black pieces, in round 4.

The main takeaway from the 2015 Sinquefield Cup is that when you put this many top GMs head-to-head with $1 million on the line - the total purse for the Grand Chess Tour - you're going to get some fireworks. And how!