Thirty-one years ago Monday, the Expos made an aggressive, win-now move for an All-Star starting pitcher. Here’s how it broke down:

May 25, 1989
Expos receive: LHP Mark Langston^
Mariners receive: RHP Brian Holman, RHP Gene Harris, LHP Randy Johnson
^RHP Mike Campbell was sent to Montreal on July 31 to complete the deal

Johnson went to Seattle and eventually blossomed into one of the greatest pitchers in Major League history. But at the time, nobody thought all that much of his inclusion in the return. He was a gangly 25-year-old who had struggled mightily in the Majors that year and was prone to bouts of wildness.

Writing about the deal in Sports Illustrated shortly after it was completed, Peter Gammons described Johnson as having, “a great arm,” but also as, “a well-known space cadet.” According to Gammons, the Expos considered Harris the best of the three pitchers in the deal. Dave Dombrowski, the Montreal general manager at the time, later told USA TODAY that some people had believed both Harris and Holman to be superior prospects.

“We liked [Johnson],” said Dombrowski, who would go on to great success running the Marlins, Tigers and Red Sox. “But nobody said we were trading a Hall of Fame pitcher.”

That’s exactly what they did, of course. But on this anniversary, it’s worth asking: What if they didn’t?

It’s hardly a far-fetched question, and the effects of such an alternate history could have been enormous. Would the Expos have gotten that elusive title shot? Would Johnson and Pedro Martínez have formed the best rotation duo of all time?

Let’s step into our time machine and take a look.

How the Big Unit stays in Montreal

This all started with Langston, who was one of the top starting pitchers in the sport in the late 1980s. But he and the Mariners -- not known for their financial muscle under then-owner George Argyros -- couldn’t come together on an extension that would have kept the lefty in Seattle past ‘89.

So it was well known that Langston was a trade candidate, with Seattle yet to enjoy a .500 season. At multiple points, it appeared the Mets would land Langston, only for Argyros to veto the deal, which supposedly would also have sent Jay Buhner to New York, and Howard Johnson, Sid Fernandez and Kevin Tapani to Seattle.

“We were about as close as you can come to making the deal on a number of occasions,” Mets vice president Joe McIlvaine said.

According to Gammons’ reporting, Argyros later killed a three-way trade that would have included Langston going to Toronto, George Bell to Atlanta and Al Leiter to Seattle. There was talk of other trade discussions as well, involving the Dodgers, Red Sox and Royals.

But in the end, it came down to New York (with a revised offer) and Montreal. The Mariners picked the latter, and after Langston turned down a last-ditch extension bid, Johnson was headed for the Pacific Northwest.

Imagine if one of those previous trades had gone through instead. It would have been a disaster for the Mariners, who without Johnson’s 1995 AL Cy Young Award campaign, likely don’t make their first postseason and defeat the Yankees in the AL Division Series -- in a triumph that is often credited with saving baseball in Seattle. (In a twist of fate, a dominant Johnson beat Langston and the Angels in the AL West tiebreaker game that October.)

The Expos would have missed out on some great pitching from Langston, which nonetheless did not prevent them from falling short of the playoffs for the 20th time in 21 seasons, or keep the lefty from leaving as a free agent. But in the long term, that setback could have paid enormous dividends.

1993 becomes the Expos’ year

It’s impossible to know what would have happened to Johnson in Montreal. He could have been traded later, sustained a career-altering injury or simply never developed into the pitcher who finished as the AL Cy Young runner-up in a breakout 1993 campaign.

But let’s assume the Expos hold on to Johnson, and that he follows the same developmental path. In that case, the Big Unit’s breakout would have been extraordinarily well-timed.

The Expos’ 1993 season came after years of disappointment that included only one heart-breaking trip to the postseason (‘81), and it came before the ‘94 strike wiped out that club’s October dreams. Felipe Alou’s ‘93 squad was great as well, led by 26-and-under stars such as Moises Alou, Delino DeShields, Marquis Grissom, Larry Walker and John Wetteland. In the final year before MLB expanded each league from two to three divisions, Montreal’s 45-22 finishing kick brought it within three games of catching the Phillies in the NL East.

Now, imagine that team with the fire-breathing addition of Johnson, whose 135 ERA+ across 255 1/3 innings produced 6.6 wins above replacement (WAR), according to Baseball-Reference. Montreal already had solid pitching, but even so, the team’s top three in terms of starts (Dennis Martinez, Ken Hill and Chris Nabholz) combined to total 7.1 WAR.

In other words, it’s not hard to see Johnson getting the Expos over the hump and into October. From there, they still would have needed to beat a 104-win Braves team, then outduel the actual champions, the Blue Jays, in an all-Canada World Series. It would have been a tall order. Fortunately, Johnson is 6-foot-10.

(The Expos also came up just two games short of the Dodgers in the 1996 NL Wild Card race. However, it’s less clear that Johnson would have helped that year, even if he were still in Montreal, due to a back injury that limited him to 61 1/3 innings for Seattle).

Randy-Pedro becomes the ultimate 1-2 punch

Less than five years after the Expos sent away a future Hall of Fame pitcher before he reached his potential, they acquired another, in a similar situation.

Pedro Martínez was not as much of an enigma as Johnson. Before the 1992 season, he was Baseball America’s No. 10 prospect. The next year, he dazzled for the Dodgers out of the bullpen as a rookie, but L.A. wasn’t sure the slight right-hander could hold up in the rotation and sent him to Montreal on Nov. 19, 1993, in order to plug their hole at second base with the coveted DeShields.

“Our staff feels he will quickly develop into a front-line starter,” said Dan Duquette, who had taken over the front office from Dombrowski.

They were right. The 22-year-old Martínez was electric from the beginning, and over his first three seasons in Montreal he was a stellar rotation piece who sometimes flashed brilliance. In 1997, Martínez ascended to an elite level, leading the NL in numerous categories and winning his first Cy Young Award.

In this alternate universe in which the Expos hang on to Johnson, see him blossom into a star and win the NL East (and perhaps more) in 1993, do they still acquire Martínez after that season? Maybe not. And even if they do, are Johnson and Martínez together for more than one strike-shortened season? Again, the answer could be no. In Montreal, Johnson may not have signed the four-year extension that kept him in Seattle beyond the ‘94 campaign.

• Big Unit or Pedro? Choose your legendary ace

But what fun is that? Let’s assume, for the sake of argument, that the Expos acquire Martínez, keep Johnson and hold on to both through at least 1997. The result that year could have been one of the greatest one-two punches any team has ever boasted:

Johnson (SEA): 213.0 IP, 2.28 ERA, 197 ERA+, 2.82 FIP, 34.2 K%, 8.0 WAR
Martínez (MON): 241.1 IP, 1.90 ERA, 219 ERA+, 2.39 FIP, 32.2 K%, 9.0 WAR

Since 1900, only six pairs of teammates -- and three since 1936 -- have produced at least 8 pitching WAR apiece. (Two of those were Johnson and Curt Schilling with the 2001-02 D-backs). The last qualifying pair with ERA+ figures of at least 190 were Carl Lundgren and Jack Pfiester of the 1907 Cubs, firmly within the Dead Ball Era. And only five pairs have finished first and second in a Cy Young Award race. (Again, including the Johnson-Schilling duo twice.)

Here is a look at 10 of the best single-season starting pitching pairs in history, factoring in value (WAR), effectiveness (ERA+) and firepower (K’s). The theoretical Johnson-Martínez duo is included.

Best ever? They at least would have been in the conversation, despite Martínez and Johnson both going on to enjoy even better seasons with Boston and Arizona, respectively.

Eventually, the two aces would become teammates, in a way. In 2015, they both cruised into the Hall of Fame on the first ballot, and became part of one of the best Cooperstown classes in history with John Smoltz and Craig Biggio.

The long term

The butterfly effect of Johnson staying with the Expos is unknowable, but potentially staggering. Would the Big Unit have helped save baseball in Montreal instead of Seattle? Would he still have wound up in Arizona, where his almost superhuman effort in the 2001 postseason brought a title to the desert and kept the Yankees from a four-peat? For that matter, would Martínez have been traded to Boston, where he put together arguably the two best pitching seasons of all time and helped the Red Sox end their infamous championship drought?

The list could go on forever, and there are no certainties.

But this much is clear: If that big trade had never taken place 31 years ago, baseball history could look quite a bit different.