It should have been one of the proudest days in Falcons annals — a poised and powerful showing on the international stage coupled with a rousing victory at a significant moment. It will sit instead on the shelf of all-time worst losses in the history of a team that has known more than its share of wretched ones.

The fourth-quarter collapse against Dallas in the 1981 playoffs. The blocked punt against the 0-13 Colts in 1986. The wasted 17-point lead against San Francisco in the NFC championship game two seasons ago. And this horror at Wembley.

“As tough a loss to take as any I’ve been a part of,” quarterback Matt Ryan said.

“You’re up 21-0,” owner Arthur Blank said. “There’s no way you lose that game — just no way. There’s nothing else I can say.”

Many among us expected the Falcons, who entered having lost four in a row, to cross the pond and spit the bit. Instead they offered their best non-Tampa Bay half of the year, a performance so comprehensive — the incredible shrinking offensive line blocked, Ryan posted a passer rating of 145.1 and the league’s 30th-best defense outdid the top-ranked unit — that it made you believe something might still be made of this season.

“This was going to be our stand,” receiver Roddy White said, and for a while it was. Then it became a fall that could, in the grand scheme, cost a lot of people their jobs. Because if you can’t win on a day when so much goes right, when can you?

Everything that went right early went wrong after halftime, though technically the slippage began before the break. After Robert Alford intercepted Matthew Stafford’s pass with 1:14 left in the second quarter and two timeouts at the Falcons’ disposal, coach Mike Smith opted not to try for more. (“We’re up three scores and didn’t want to turn it over,” he said. “That was my decision.”)

With 4:48 left in the third quarter, the Falcons still led by 18. Then Golden Tate slipped past cornerback Desmond Trufant and by safety Kemal Ishmael and caught 59-yard touchdown pass on third-and-25. The Lions, who’d done next to nothing, had life.

Then Ryan, dodging left, sought to throw right. It was as bad as interception as he has ever delivered, and only Julio Jones’ speed in catching Cassius Vaughn kept the Lions from closing within 21-17. But the game, which had been one-way traffic in the first half, had been rerouted the other way.

The Lions scored a touchdown with 4:01 left to draw within two. Trufant fought through a pick to thwart the two-point try to Tate. A splendid catch by Harry Douglas on third-and-5 gave the Falcons one first down and a screen to Jones yielded another. All that was left was to do nothing silly. They couldn’t manage even that.

On second-and-9 with 1:55 remaining and the Lions out of timeouts, a Falcons lineman — the official play-by-play identified a defender as the violator, which couldn’t have been true — was flagged for holding. That stopped the clock, saving 35 seconds for the Lions. On third-and-9, Jones dropped the ball on another screen. (“I should have just caught it and went down,” he would say later.) That saved another 35.

The Lions took the ball at their 7 with 1:38 remaining, at least a minute more than they could have hoped. They moved into field-goal range, though not point-blank range. (Detroit hadn’t made one between 40 and 49 yards this season.) Smith called two timeouts, apparently trying to save seconds for the offense he didn’t let try to score at the end of the first half, but all he did was give the Lions wiggle room. Paul Soliai was flagged for defensive holding to hand the Lions a first down. Then everything went crazy.

At 0:04, Matt Prater tried a 43-yard field goal and missed — but wait. The Lions were flagged for delay of game. Since the clock was stopped, there was no 10-second runoff, which would have made the Falcons winners. Prater dropped back five yards and kicked it true.

Said Jones: “We’ve got to find a way to win. We’re up 21-0, and we found all the right ways to lose.”

As bad as the previous five losses had been, this was worse. The Falcons played as well as they’re apt to play in the first half but mustered 80 yards and no points thereafter. They failed at clock management. They broke down defensively. They cast a silly interception. They’re 2-6 facing the grimmest bye week this franchise has ever known.

The most famous bit of sports commentary in Wembley history was Kenneth Wolstenholme’s description of the final seconds of the 1966 World Cup final, won by England over West Germany. “Some people are on the pitch,” Wolstenholme intoned. “They think it’s all over.” Then Geoff Hurst added the clinching goal on the match’s last kick, prompting this: “It is now!”

The Falcons will put up a stiff upper-lip and speak of the continuing weakness of the NFC South, but for this team and this administration, I think it’s all over.