There’s a reason it took more than a quarter of a century for someone to win the UEFA Champions League in back-to-back seasons. AC Milan won the old European Cup in 1989 and 1990, and then nobody repeated again until Real Madrid won it three years in a row from 2016 through 2018.

Certainly, there were dynastic runs, like Barca winning it three times in six years, or Real winning three in five at the turn of the century. But winning it back-to-back is the rarest of things.

On Tuesday, as the Champions League returned from its two-month hiatus to kick off the typically unchained knockout stages following the dreary group stage, Liverpool once again learned why repeating in Europe is so tough.

It’s the margins – margins so fine they make a brand-new razor blade look like a blunt instrument.

The margin of a ball taking a bounce right in front of Saul Niguez from Atletico Madrid’s corner, allowing the midfielder to slide it into the net for the only goal in the first leg of the round of 16 with the defending champions, which ended 1-0. The margin of Atletico not quite converting any of its other early chances, in one case saved by Andy Robertson’s outstretched toe.

The margin of Mohamed Salah’s shot just skimming off a defender and over at the other end. Or the Liverpool forward’s open header only just missing the target right after halftime. The margin of Alvaro Morata just getting nudged off the ball as he wound up for a wide-open shot to put away the game – and likely the two-leg tie. The margin of Jordan Henderson volleying the ball inches wide.

A narrow margin was all Atletico’s embattled manager Diego Simeone needed. He’s leveraged those scarce victories into two runs to the Champions League final, in 2014 and 2016, turning an unappreciated and unloved club into a European power. There was little pretense of Atletico even attacking for much of the second half, only venturing out when Liverpool began to tire of crashing onto the red-and-white embankment. All the while, Simeone waved wildly at the fans to ratchet up the pressure in the cooker.

Simeone can turn a 1-0 score into anything, like water into wine – or maybe vinegar, more like. In the return leg in Liverpool, the Reds will huff and puff and struggle to blow down the Atletico defenses, poured from reinforced concrete. But there isn’t a man in the world more qualified to protect a 1-0 lead.

And so Liverpool’s dream season could finally unravel, at least partially, in that single moment in front of its goal in the Wanda Metropolitano. Everything had broken right for Jurgen Klopp’s much-loved, much-admired team this season. It couldn’t have been in a rosier position going into this game. Its staggering 25-point Premier League all but sews up a first league title in three decades as Manchester City was wasteful and everybody else was just a mess.

The storied club didn’t even bother to field a first-team lineup for an FA Cup replay, instead sending the under-23s and opting to keep its winter break pristine. It was betting it all on that liberating league title and the European title defense, with the Club World Cup already secured. And it could rotate players in the league to rest and prepare for the Champions League optimally.

But then Liverpool fell behind for the first time in 1,739 minutes, if you discount the Dec. 17 League Cup match against Aston Villa, when the under-23 team was eliminated as well. Atletico was only the fourth team all season to shut Liverpool out – in 43 games – during regulation. If you discount the Villa game or the Club World Cup final against Flamengo, when Liverpool won in extra-time, it was just the second shutout. The first also came in the Champions League, in Liverpool’s very first group stage match against Napoli on Sep. 17, the only other first-team defeat of the season.

During the second half, Klopp grew ever more animated, grimacing and getting cautioned by the referee. He knew the trap his team had walked into. He knew that Simeone had organized his side so meticulously that there was simply no room for Liverpool’s lightning-quick transitions. Every time the Reds won the ball, Atletico was already there, bunkered into its half, tight lines arranged just so, drilled to perfection. You can’t catch out a team that never leaves its half, pouncing on a set piece for the lead.

Klopp knew what the margins were. He understood immediately how much trouble his side is in. He now knows how close it is to seeing its superb accomplishments in the league sullied somewhat by a European campaign that could collapse prematurely.

Leander Schaerlaeckens is a Yahoo Sports soccer columnist and a sports communication lecturer at Marist College. Follow him on Twitter @LeanderAlphabet.

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