Disappointed Brumbies coach Dan McKellar said his team had the belief they could push all the way to the Super Rugby title but failed to adapt their game under enormous pressure from the outstanding Jaguares.
The Argentinians booked their place in a maiden decider with the 39-7 victory and will face the winner of the Crusaders-Hurricanes clash in Christchurch this afternoon.
McKellar seemed at a loss to explain how the Brumbies turned over so much possession in the first half and failed to adapt to the Jaguares' constant pressure, especially at the lineout.
Appearing nervous and rattled from the start, the Brumbies were not allowed to find their rhythm, with the Jaguares punishing every error and continuing to ratchet up pressure.
"We turned over too much possession, our lineout let us down in the first half and failed to build any pressure or have any field position," McKellar said.
"We turned it over at lineout time, took some poor options off the back of the scrum, we just gifted them an easy try to start with.
"But (the result) comes down to our inability to hold possession and build any pressure."
The lineout has been a particular strength for the Brumbies this season, providing a platform to launch their feared rolling maul and second-phase attack.
But they were unable to find rhythm, with poor throws and jumps as the Jaguares took them on, eventually causing a crisis of confidence for hooker Folau Faingaa and his forwards.
"Defensively they came with a plan and it worked well and we were a little bit rattled there and probably didn't adapt as well as we could have."
The Brumbies trailed 20-0 after a calamitous first quarter, although McKellar felt his side had a chance to fight back in the second half after a try just before the break.
"I thought the try just before halftime was a good result for us, we were in a good position at 20-7 because clearly they were the dominant team in the first half," he said.
"We just needed to get some possession and field position and we got that just after halftime and won the lineout but dropped the ball.
"And that was the story of our night. We turned the ball over and then we gifted them a try trying to put a little kick in behind and they went the length of the field."
While the Brumbies felt they could have gone all the way, McKellar said the Jaguares were a great chance to notch their first title regardless of who they played in the decider.
"We certainly feel we could have gone all the way through but tonight we weren't good enough and you've got to give the Jaguares a bit rap there, I thought they were really good," he said.
That's the reality, sometimes you've just got to take your hat off to your opponent and accept that they were the better side.
"I think they're a very well-rounded team.
"I think in the past Argentinian teams have been very scrum-related, very forward dominated game, whereas now they've got a lot of skill - a lot of footwork, a lot of dangerous players in their backline.
"And I think the Jaguares, will (be in contention) whoever they play in the final next week and Argentina will be difficult for teams at the World Cup."
The difficulty will be overcoming the travel if they have to head to Christchurch to take on the Crusaders.
"Travel is the difficult part. We had to deal with the travel this week, so if they've got to travel to Christchurch next week, it's obviously limited training time and a big focus on recovery and that's going to be a challenge they've got to deal with," he said.
Jaguares coach Gonzalo Quesada said making the final was the culmination of years of work and while it was an amazing feeling, there was still a job to be done.
"It's an amazing feeling but we're going to try not to get too distracted," Quesada said.
"We are enjoying this moment because we have to respect these moments and enjoy them."
The Brumbies' Super Rugby title challenge is over after a disappointing 39-7 loss to the Jaguares in Buenos Aires.
After winning their past seven games in succession, the Brumbies had hoped to brush aside the challenge of travelling halfway around the world to charge into their first final since 2013.
But they saved one of their worst performances of the season for the semi-final, with the Jaguares storming into their first Super Rugby decider.
The Argentinians will take on the winner of today's other semi between the Crusaders and Hurricanes and could yet host the final if the Canes are able to cause an upset in Christchurch.
It was the Brumbies' biggest ever loss in a finals match and left the players' shattered at an uncharacteristic performance in which they failed to perform at their best in any facet of the game.
With their vaunted lineout breaking down early in the game, the Brumbies were under massive pressure and were suffocated by the Jaguares, who dominated possession and field position and were outstanding in defence, with the Brumbies struggling all game just to get across the advantage line.
On the backfoot all match - thanks in large part to a through a tactical kicking masterclass from flyhalf Joaquin Diaz Bonilla - the Brumbies were unable to inject the likes of centres Tevita Kuridrani and Irae Simone, who have caused such headaches for opposition defences all season, while the magnitude of their deficit in the second half left them chasing points and taking risky options that ultimately backfired.
Brumbies captain Christian Lealiifano, who played his 150th and final match for the club in the semi-final, was shattered after the match.
"It was really disappointing that one," Lealiifano said.
"They were very good tonight, they put us under a lot of pressure and we just couldn't get back on top.
"I felt like the game was turning (when) we got a foot in it late in that first half but the Jaguares were very good tonight."
Lealiifano, who will leave the Brumbies to play in Japan next season said the loss marked "a very sad time".
"The Brumbies will always be in my heart forever, they've been a special club to me.
"It's a disappointing way to go down but that's the way rugby is. We'll take some time to soak it in and enjoy now."
Jaguares lock Guido Petti, who overcame an ankle injury to play, said his team had been confident heading into the semi.
"We knew this was going to be the match that we worked for all year and we came out from the first minute to build and the match came for us," Petti said.
A try after the halftime hooter gave the Brumbies some hope but they face a massive task to fight back into their semi-final against the Jaguares, trailing 20-7 at the break after a disastrous start.
The visitors were down 20-0 after just 20 minutes following a disastrous start but fought back in the dying minutes of the half, with Folau Faingaa diving over to score one of the most important tries of the season.
The Brumbies needed a strong start to settle their nerves and take the vocal home crowd out of the equation but had a horror opening, going down 13-0 inside the opening 13 minutes.
A mix-up in communication from Rob Valentini and Tony Pulu after Jaguares flyhalf Joaquin Diaz Bonilla lobbed a chip kick inside the 22 allowed the ball to spill loose, with former Brumby Tomas Cubelli diving on the pill to score after just three minutes.
Diaz Bonilla slotted the conversion and then a pair of penalties - one for a high tackle and another for offside play - as the Brumbies struggled to get into the match.
The lineout has been the Brumbies' bread and butter this season, providing a platform for their rolling maul, but it was ineffective early, with missed throws and simple errors increasing the pressure on the Brumbies.
The Jaguares though were making the most of their opportunities.
They turned the screws further with a second try in the 18th minute, lock Tomas Lavanini diving over after a well-executed set play from a lineout in the Brumbies' 22, with Diaz Bonilla's conversion giving them a 20-0 lead.
With just 20 per cent possession in the first 20 minutes, the Brumbies were on the back foot continually.
And they failed to make the most of their limited chances, their lineout problems continuing when they finally won a penalty to make only their second foray into the Jaguares' territory in the 25th minute.
But they finished strongly to give themselves a glimmer of hope for a second-half miracle.
There were good signs early in the second term for a Brumbies team desperately needing to score first.
Tom Banks stepped up to thump a relieving kick inside the Jaguares 22 for a lineout that ordinarily would have provided the platform for a slick attack.
But they squandered possession with another simple error and were soon made to pay dearly.
When Christian Lealiifano's attempted to grubber through the defence, it rebounded off a Jaguares foot and winger Matias Moroni was able to race away before drawing the defence and slipping an inside pass for Matias Orlando to score.
The pain continued when Orlando bumped off a couple of paltry defensive efforts to race away for his second try, improving his position in the in-goal to score under the posts, with the conversion leaving the Brumbies trailing by 27 points with less than 20 minutes remaining and the game out of the Brumbies' reach.
Fullback Emiliano Boffelli put the icing on the cake with a try in the 77th minute to send his team to a maiden final.
Tries: Orlando 2, Cubelli, Lavanini, Bofelli
Cons: Diaz Bonilla 4
Pens: Diaz Bonilla 2