Way back in early 2019 — before the days of NIL, the transfer portal and a global pandemic — Clemson beat Alabama to become the first team to finish 15-0 since Penn in 1897.

In the future, the expanded College Football Playoff may result in a national champion that goes 16-0 or even 17-0. Doing so would be a monumental achievement requiring going through the regular season unscathed and then beating three or four of the best teams in the country.

Nineteen Bowl Subdivision teams are still chasing immortality heading into Week 6, led by the top four teams in the US LBM Coaches Poll: No. 1 Texas, No. 2 Alabama, No. 3 Ohio State and No. 4 Tennessee.

Based on history, the odds are that these unbeaten records won't last long. Here's our best guess for when each of the current unbeatens suffer their first loss of 2024.

No. 1 Texas (5-0)

First loss: vs. No. 5 Georgia, Oct. 19.

Look for the Longhorns to sail past No. 17 Oklahoma on Oct. 12 given the Sooners' offense state of affairs. Getting Georgia at home definitely helps, but the Bulldogs are heading into October deeply motivated to move past last Saturday's setback at Alabama and end the regular season with just one loss.

No. 2 Alabama (4-0)

First loss: at No. 4 Tennessee, Oct. 19.

This is the first of three ranked opponents in a row for the Crimson Tide, with the Volunteers followed by No. 9 Missouri in Tuscaloosa and No. 12 LSU on the road. While Alabama could definitely stay perfect through this run, Tennessee looks like an elite team capable of making the most of the home-field advantage in this rivalry.

No. 3 Ohio State (4-0)

First loss: Sometime in postseason play. (Maybe.)

The Buckeyes are beginning to resemble a powerhouse built to run through a tough Big Ten slate that includes road trips to No. 6 Oregon and No. 7 Penn State. Both teams could trip up Ohio State, as could No. 10 Michigan to end the regular season. Here's guessing the Buckeyes manage to run the table until the playoff.

No. 4 Tennessee (4-0)

First loss: at Georgia, Nov. 16.

The second of two high-profile games on its SEC docket, the Volunteers might be able to get past Alabama at home but will be tripped up by the Bulldogs in Athens. Given that the rest of the schedule is very manageable, splitting those two games should send Tennessee to the conference championship game and guarantee at least an at-large playoff bid.

No. 6 Oregon (4-0)

First loss: vs. Ohio State, Oct. 12.

While drawing the Buckeyes in Autzen Stadium is a huge bonus, Ohio State represents a different sort of test for a team and program still getting acclimated to life in the Big Ten. Whether Oregon can force a rematch in the Big Ten championship might hinge on a road trip to Michigan on Nov. 2.

No. 7 Penn State (4-0)

First loss: vs. Ohio State, Nov. 2.

There's a trip to No. 15 Southern California on Oct. 12 that could prove difficult and a possible trap game at Wisconsin the week before hosting the Buckeyes, though the Badgers probably lack the firepower to keep pace with Penn State. History says the Nittany Lions will lose to Ohio State, winners of seven in a row and 11 of 12 in the series.

No. 8 Miami (5-0)

First loss: at No. 22 Louisville, Oct. 19.

There's a loss lurking somewhere on this schedule as shown by the home struggle against Virginia Tech that had a controversial ending. The toughest road trip will be at Louisville later this month, so let's go with the Cardinals winning a shootout against Cam Ward and the vastly improved Hurricanes.

No. 9 Missouri (4-0)

First loss: at Alabama, Oct. 26.

After needing double overtime to get past Vanderbilt its last time out, Missouri's first loss could come as soon as Saturday's game at No. 21 Texas A&M. If not there, the Tigers will likely be heavy underdogs in Tuscaloosa to end the month.

No. 16 Iowa State (4-0)

First loss: vs. Central Florida, Oct. 19.

The Cyclones could run the table until facing No. 18 Utah and No. 20 Kansas State to end November. While it was just blown out at home by Colorado, UCF has the team speed and running game to knock off the Cyclones in Ames.

No. 19 Brigham Young (5-0)

First loss: vs. Oklahoma State, Oct. 19.

Arizona could sneak up and knock off BYU on Oct. 12 given how the Wildcats were able to smother Utah. The better pick is Oklahoma State a week later, though the Cowboys have to first figure out a way to get running back Ollie Gordon back on track.

No. 23 UNLV (4-0)

First loss: vs. Syracuse, Friday.

The Rebels looked great against Fresno State with quarterback Hajj-Malik Williams starting in place of Matthew Sluka. But Syracuse is a solid Power Four team and the Rebels' toughest opponent to date. If not Friday, look for Boise State to win at UNLV on Oct. 25.

No. 24 Indiana (5-0)

First loss: vs. Washington, Oct. 26.

Curt Cignetti's Hoosiers clearly have the scoring punch to get past Northwestern and Nebraska to continue the program's best start since 1967. The Huskies are better than their 3-2 record suggests and will be a very tough matchup for Indiana even with the game at home.

Army (4-0)

First loss: vs. East Carolina, Oct. 19.

Army's three FBS defeated opponents are a combined 4-11 and its next two, Tulsa and Alabama-Birmingham, are among the worst teams in the country. So how good are the Black Knights, really? ECU has turned the page on a dreadful 2023 season and should end up around seven wins in the regular season.

Duke (5-0)

First loss: at Georgia Tech, Saturday.

Duke has wins against Northwestern, Connecticut and North Carolina by a combined 12 points, giving the strong impression a loss is lurking somewhere around the corner. That may be as soon as Saturday at Georgia Tech, which is about a touchdown favorite.

James Madison (4-0)

First loss: vs. Coastal Carolina, Oct. 10.

James Madison started slowly but has stepped on the gas in wins against North Carolina (70-50) and Ball State (63-7). Going unbeaten and reaching the playoff is a real possibility. But of the matchups still to come, Coastal Carolina and Georgia Southern one week later look like the biggest hurdles in the way of 12-0.

Liberty (4-0)

First loss: vs. Western Kentucky, Nov. 23.

Liberty has played only one team of any consequence (East Carolina) and won't play another until hosting Western Kentucky in late November. The Hilltoppers have a great win against Toledo and a very close loss to Boston College, painting the picture of a team that could win nine or more games to reach the Conference USA title game.

Navy (4-0)

First loss: vs. No. 13 Notre Dame (in East Rutherford, N.J.), Oct. 26.

The Midshipmen could lose this weekend to Air Force given the nature of this rivalry, though the Falcons have been absolutely awful through one month. If it gets past Saturday, look for Navy to roll over Charlotte before getting handed a seventh loss in a row against the Fighting Irish.

Pittsburgh (4-0)

First loss: at North Carolina, Saturday.

Picking the Tar Heels to beat anybody feels wrong given how it has imploded against James Madison and Duke. But after sneaking past Cincinnati (28-27) and West Virginia (38-34) in recent weeks, the Panthers are unlikely to stay perfect in what has shaped up to be a must-win game for North Carolina.

Rutgers (4-0)

First loss: at Nebraska, Saturday.

Like Pittsburgh, Rutgers was very fortunate to beat Washington last weekend after giving up 7.1 yards per carry and being outgained by more than 200 yards. The key to winning in Lincoln will be to force turnovers, but that's something the Cornhuskers have been able to avoid in starting 4-1. Nebraska has committed just three giveaways after turning the ball over an FBS-worst 31 times a year ago.