At halftime, one could almost be forgiven for thinking that Jim Mora would be fired, Josh Rosen should think about sitting out the rest of the season and save himself for the NFL and UCLA was on its way to a horrific start to another disappointing season. The Bruins' play for much of their 2017 opener against Texas A&M truly was that bad.

But let the lesson be learned yet again: Never make definitive conclusions after one half of a college football game. UCLA is laughing at all of us now.

Rosen led UCLA back from a 34-point deficit to beat Texas A&M 45-44 at the Rose Bowl in Pasadena on Sunday, the second-largest comeback in the history of college football. Only Michigan State's comeback from down 38-3 against Northwestern in 2006 tops it.

With 4:08 left in the third quarter, Daniel LaCamera made a 48-yard field goal to put Texas A&M ahead 44-10, an insurmountable lead in almost any situation imaginable, even though true freshman Kellen Mond had just replaced the injured Nick Starkel at quarterback for the Aggies. After all, UCLA's offense looked like it had just as many problems as last year's 4-8 team, as the offensive line struggled to protect Rosen, receivers didn't get open and the running game offered little help.

Instead, in his first game since Oct. 8, 2016 -- he missed the final six games of last season with a shoulder injury -- Rosen finally looked every bit like the future No. 1 NFL Draft pick he's been touted as since he was in high school.

At least he did in the final 19 minutes.

With 2:06 left in the third quarter, Soso Jamabo finished off a 75-yard UCLA drive with a six-yard touchdown run to cut the lead to 27. After Texas A&M went three-and-out, Rosen led the Bruins on an 85-yard drive, capped by a nine-yard TD pass to Darren Andrews. The Aggies pinned UCLA at the four-yard line on a punt, but it took only five plays for the Bruins to march down the field, with four straight Rosen completions of 28, nine, 17 and 42 yards, the last a miracle TD pass to Andrews that should have been intercepted.

It was exactly the kind of luck a team needs to complete such a monumental comeback:

With the help of UCLA penalties, Texas A&M drove into the red zone, but Mond was sacked on a third-and-four, and Braden Mann -- who replaced an injured LaCamera -- missed the subsequent field goal. Trailing by 13, UCLA got the ball back at its own 26, and it needed only six plays to go 74 yards, with Rosen finding Theo Howard for a phenomenal, off-balance 16-yard touchdown pass under pressure.

UCLA still needed one more stop to have a chance to win, and it did so with ease. On third-and-eight, Mond was sacked by Jacob Tuioti-Mariner and five-star true freshman Jaelan Phillips, who lived up to his No. 1 recruit billing in his first game.

Rosen and the Bruins miraculously had plenty of time left to march down the field. They got to the Texas A&M 20, where they faced fourth-and-six. Rosen dumped it outside to Jamabo, who ran 10 yards for the first down. The next play, Rosen faked a spike and threw it up to Jordan Lasley, who hauled it in. J.J. Molson added the go-ahead PAT.

UCLA trailed by 34 with 4:08 left in the third quarter. With 43 seconds in the fourth, it held a one-point lead, a 35-point turnaround in 18 minutes, 43 seconds. There were no turnovers, defensive touchdowns or short fields to aid the Bruins' comeback, either. Their shortest touchdown drive of the final five was 66 yards. In total, the five comeback TD drives covered 396 yards.

The Bruins still needed yet another stop to prevent an all-time great comeback from turning into an agonizing last-second loss. The defense came up big, again. Mond threw three incomplete passes, and on fourth down he ran for it but was ruled just short of a first down by instant replay, sealing the most improbable win in UCLA history and one of the most improbable wins in sports history.

Rosen completed 35 of 59 passes for 491 yards and four touchdowns with no interceptions. He had 292 yards in the fourth quarter alone. He had three receivers finish with more than 100 yards: Caleb Wilson had 15 catches for 208 yards, Andrews had 12 catches for 142 yards and Lasley had four for 100 yards. He finished with these numbers after such an ugly start, at least after an opening field goal drive.

Following that field goal, UCLA lost two fumbles, then had four straight three-and-outs. When the Bruins finally got something going with a big Rosen throw to set up a short TD run, Texas A&M's Trayveon Williams responded with a 61-yard touchdown run on the Aggies' next play, giving them a 38-10 lead at halftime.

Williams dazzled with 203 rushing yards, and Keith Ford added 114 on the ground. But the Aggies' persistent passing woes -- combined with the complete collapse of their defense -- helped make the Bruins' comeback possible. Starkel, a redshirt freshman, started and completed 6 of 13 for 62 yards. After his third-quarter injury, Mond came in and rushed for 54 yards but completed just 3 of 17 for 27 yards.

The UCLA defense stepped up after a disastrous first half, and the offense got the long-awaited breakthrough it needed far more suddenly than ever could be imagined. In the debut of new offensive coordinator Jedd Fisch, Rosen looked tentative early behind a still-problematic line, but, remarkably, he re-gained confidence in the second half facing such a huge deficit and could seem to do no wrong.

Yes, UCLA did get a lucky a few times. Rosen also played one of the greatest fourth quarters any quarterback has ever played, showcasing confidence, poise, a big arm and ability to make big throws in pressure situations. Amid all the noise about his previous on-field results -- promising but not yet spectacular -- and off-the-field thoughts, this was the Rosen we had long heard about, returning from an injury to turn a total nightmare into the most memorable game of his career.

Last year in the opener at Texas A&M, UCLA trailed 24-9 deep into the fourth quarter. It scored 15 points in the final five minutes to force overtime, only to lose anyway, when a Rosen pass fell incomplete on the final play after the Aggies had already scored on their possession. This time, the odds of a comeback were far greater, and yet the result flipped. UCLA finished the job.

We won't know the big picture meaning of this game for weeks, when we get a fuller view of just how good both the Aggies and Bruins are. It could help decide a major bowl spot. It could be a thriller between mediocre teams like last year's Texas-Notre Dame opener. It could launch a Rosen Heisman campaign. We don't know.

All we know now is that we witnessed an embattled potential star quarterback play like the biggest star in the sport in the fourth quarter in Pasadena, delivering the type of comeback win that will never be forgotten, regardless of the long-term stakes.

For a few hours Sunday night, there was no reason to believe in UCLA. By the end of the night, Rosen and the Bruins delivered an impossible result that proved all of us wrong.

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Contact Matt at matt.brown5082@gmail.com and follow him on Twitter @MattBrownCFB and Facebook.