We need to be careful. Atlanta United is, by definition, an Atlanta team, and we all know what Atlanta teams do within sight of a championship. Ask Arthur Blank how much his other team enjoyed its night in Houston. And fat leads of a two-leg soccer series have been overturned. Ask Paris-Saint Germain about its night in Barcelona.

That said, United is in Position A to win the Eastern Conference finals and play host to the MLS Cup final at Mercedes-Benz Stadium on Dec. 8. Its 3-0 dissection of the New York Red Bulls in Leg 1 on Sunday was something to behold. (Which is what we were saying about the Falcons in Super Bowl LI when it was 28-3, but enough with that.)

“This is halftime,” United goalkeeper Brad Guzan said. “We can’t get ahead of ourselves. But this was a massive performance.”

The Red Bulls are an excellent club. They finished with 71 points, most in MLS annals. United finished with 69, tying it for second-most ever. The Supporters’ Shield – awarded the team that finishes the regular season with the most points – wasn’t decided until the final Sunday. United lost 4-1 at Toronto. The Red Bulls beat Orlando 1-nil; had they lost 1-nil, they’d have been runners-up.

In most leagues around this wide world of football, the regular season is what matters. England has its FA Cup and Spain its Copa del Rey, et cetera, but those are tournaments staged as league play is ongoing. There’s no qualifying for postseason because there is no postseason. The MLS being U.S.-based, it has appropriated the look of other U.S. sports: The bigger prize comes at the end of what’s known in soccer as a knockout competition.

United hasn’t been in existence long, but the Red Bulls have already become what Florida in Jacksonville was to Mark Richt. Entering Sunday’s match, United had met the RBs four times, winning none, drawing one. This was the opponent that spoiled United’s giddy inaugural game. On March 5, 2017, before a frothing gathering of 55,297 at Bobby Dodd Stadium, the expansion team took a 1-0 lead on Yamil Asad’s goal. The Red Bulls scored two goals in six second-half minutes, the latter being deflected into the Atlanta net by an Atlanta player, to win 2-1.

The teams played twice this season, for most of which United seemed the class of MLS. The Red Bulls won 3-1 here in May and 2-0 there in September. Never mind that the New Yorkers were working with a different coach the second time. Jesse Marsch left in July to become the assistant manager of Leipzig in the Bundesliga – the MLS is fine, but the big European leagues remain the place to be, even as a No. 2 – and was replaced by assistant Chris Armas. The Red Bulls were 12-3-3 under Armas; they’d been 10-4-2 under Marsch. In sum, they’ve been consistently good.

The Red Bulls were, however, clearly second-best Sunday. (Said United coach Tata Martino: “We were finally able to beat them.”) The home side did nearly all the early attacking, United hitting long diagonal balls to their wide men, who would them hoist crosses. The Red Bulls were content to defend. Neither team managed a shot on goal until the 32nd minute, when Josef Martinez, the league’s leading scorer, put a shot in the goal.

The catch with allowing the other team to do all the attacking is that you can’t misplay even one ball in the box. Tim Parker, a Red Bulls’ center, had headed everything in his area out of harm’s way – until he didn’t. Jeff Larentowicz, a United midfielder, lifted a cross on which Parker whiffed. Martinez chested it down and slotted it past goalkeeper Luis Robles.

We pause for a brief tutorial on two-leg soccer ties, as they’re known. A home team that has taken the lead in Leg 1 must be careful. Owing to the away-goals tiebreaker – there’s no rough equivalent in baseball/American football/basketball/hockey – it’s far better to face Leg 2 on the road after winning 1-nil than 2-1. A team facing a one-goal deficit but carrying an away goal has only to win 1-0 at its place to advance.

The second half had scarcely begun before the Red Bulls appeared to have their away goal. A set piece yielded a header that fell to the distinguished striker Bradley Wright-Phillips, who swept it past Guzan. But VAR – video-assisted referee – ascertained that New York’s Alex Muyl, who hadn’t touched the ball on the sequence, nonetheless was blocking Guzman’s view from a “passively offside” position. No goal, VAR decreed.

Come the 71st minute, United had a second goal. Miguel Almiron slipped a pass to Julian Gressel on the left side. Gressel sent a ball into the box that Martinez let run to Franco Escobar, charging in on the right. Escobar hammed it past Robles. This sublime bit of interplay put the newly minted holders of the Supporters’ Shield in a hole. A 2-1 road loss wouldn’t be awful; 2-nil would make winning on aggregate, at least in regulation time of Leg No. 1, problematic.

The Red Bullls pushed forward, leaving the field spread in a way it hadn’t been. Hector Villalba nearly made it 3-0 but hit the post in the 90th minute. Then, just after Guzan saved the visitors’ one and only shot on goal, Villalba slipped a laser just inside the post he’d just rattled. Now it WAS 3-nil, and here we did the math.

If United can score even once in Leg 2, the Red Bulls will need five goals to win. As solid as they are, they aren’t Barcelona. Atlanta United blowing this wouldn’t be the biggest flop in our city’s history -- we are Atlanta, after all -- but it’s tough to imagine this team falling on its face. This team is really, really good.