It was a pair of self-inflicted wounds that eventually, ruthlessly killed the legend of Dos-a-cero. It was malpractice, and overconfidence, and Jurgen Klinsmann doing Jurgen Klinsmann things that ended a streak begun on a February night nearly 16 years ago.

It was 2-1 Mexico on Friday in the Hexagonal opener, and they're heading back home with three points while the US ponder their first World Cup qualifying home loss to their southern neighbors in 44 years.

Klinsmann forgot the Hippocratic Oath of coaching: First do no harm. And there’s no way to pretend that coming out in a 3-5-2 — a formation that the US has played for just 45 minutes in the last five years, and never in a game that matters — didn’t lead to the first Mexico goal.

"Disjointed is the word I'd use for this start for the US,” is how FS1 color commentator Stuart Holden put it. “Often times that's what happens when you go with a new formation."

Here is the game's first goal:

First: Michael Bradley has to win the ball, but if he doesn't, someone has to be in position to support him. And a team that's played together in the same formation, and with the same personnel would understand that this is Bradley's primary weakness as a d-mid: He's not great at digging the ball out in these 1-v-1 situations. He's really, really good at reading the game and shielding the backline, and he's excellent at disrupting attacking players just as the ball is received, and he's good at stopping opposing attacks.

And that's what happened there -- he literally stopped the Mexico attack! 

But there was no one nearby to help him, so here we have asked to defend in Zone 14 all by himself, asked to stop a 1-v-2 because Timmy Chandler doesn’t track his man, and Omar Gonzalez doesn't step up, and Jermaine Jones isn’t close enough to even be part of the play.

That’s the whole point of building chemistry — you have to learn how to play together, how to cover for each other, how to form partnerships that make each player better. Players who play together, a lot, in familiar positions and familiar formations have each others' backs. Players who don't run around trying to put out fires that they have little chance of anticipating in the first place.

“Disjointed” is maybe not a strong enough word.

Things got better with the switch to the 4-4-2, because of course they did. The US have been reliably very good in the 4-4-2 and literally everybody on the field in Red, White & Blue played better in the second half. The numbers bear that out:

Klinsmann, in his postgame presser, was right to lament his team's finishing. Gonzalez had a header he should have buried, and both Jozy Altidore and Bobby Wood forced big saves out of Alfredo Talavera. Altidore's hold-up play was particularly devastating, as is readily apparent on the lone US goal of the night. If there's a silver lining to be taken from the first home loss in a World Cup qualifier for the US since Christian Pulisic was two years old, it's that Altidore and Wood have real chemistry.

But then... why, in the 89th minute, when defending a corner kick, is there no man on the back post? Why are Altidore and John Brooks apparently confused as to whether they’re playing zonal or man-to-man when defending this set piece (worth noting: Klinsmann said specifically that Rafa Marquez was Brooks' man on the play, and I'll add that Altidore obviously has to do better -- regardless, and in a vacuum -- protecting that near post)? Why were basic precautions not taken? Why is no one on the same page?

Was none of this covered in training?

Mistakes on set pieces happen, and Brooks has earned a lot of equity for the US with his play over the past year. He was mostly very good this evening, and was spectacular this summer at the Copa America.

Mistakes preparing for set pieces, though? Forgoing a man on the back post when the whole team has dead legs? That is on the coach. Having a guy there means having a damn good chance of clearing Marquez's shot, and not having a guy there is a failure of planning.

There's only one person responsible for that.

There are no happy answers here for the US, just hard questions and an unhappy loss. Dos a cero is dead, the Hexagonal has just started, and everyone is right to feel legitimate frustration at both what was and what should have been.

A few more things to ponder:

5. Klinsmann called the formation a 3-4-3, but if that's the case it was unlike any 3-4-3 I've ever seen. It was a 3-5-2 with Pulisic as the No. 10 underneath Altidore and Wood, and pretty bog-standard at that.

Pulisic was poor in that role (note: This does not mean that I think he is a poor player, or that he will never be a good No. 10, or that he shouldn't be a starter). He was really, really good after the half hour mark when moved to the wing -- which is where he plays for his club team, Borussia Dortmund.

4. The 3-5-2 is a wonderfully useful formation if you use it regularly, as Toronto FC are currently showing in the MLS playoffs. But trying it in a qualifier against Mexico was suicidal.

3. Mexico made hay by going right at Chandler and Gonzalez, which was obvious from the start:

You can't play the 3-5-2 if you don't have center backs who are comfortable defending in space. Omar was okay, but he needed a ton of help -- and that's part of why the US constantly had gaps in midfield.

2. Jones and Bradley had another day of neither one really making the other better.

1. While the shift to the 4-4-2 was a big reason for the shape of the game changing, it has to be noted that Andres Guardado came off injured in the 28th minute and in his absence Mexico had zero rhythm. The man literally did not miss a pass during his time on the pitch.

Is completion percentage everything? No. But I'm a big believer that passing the ball cleanly and in a rhythm matters. El Tri don't have anyone else of Guardado's caliber for that role.

Whoever has that guy on their team has got a really, really good midfielder.