It has been four seasons since Desmond, 33, last dressed in the home clubhouse here. Yet the lessons he learned apply not only to the young Rockies around him but to Nats teams past and present. Don’t get comfortable, whether you’re player or fan, because nothing in baseball is permanent.

“I throw out the warning all the time: Hey, man, I hope you guys are enjoying this, this core group of guys, while it lasts,” Desmond said Tuesday afternoon. “There’s no promises that this group is going to stick together forever.”

There was a time, and it doesn’t seem that long ago, when the core of the Nationals would have been easily identified as Ryan Zimmerman, Jayson Werth, Jordan Zimmermann, Gio Gonzalez, Stephen Strasburg, Bryce Harper and Desmond — with a bullpen of Craig Stammen, Tyler Clippard and Drew Storen. In 2012, that group propelled the Nats to their first National League East crown. Four more years? Heck, keep ’em together forever.

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And then came the offseason. Sean Burnett, a lights-out lefty, signed with the Los Angeles Angels. Michael Morse, a fan favorite slugger, was traded to the Seattle Mariners. It’s baseball business, sensible stuff that seems trivial now. But the clubhouse can be a fragile place. And it doesn’t always recover.

“Guys that we thought would be part of the core forever,” Desmond said, “they start to fade away.”

This isn’t to be wistful about players whose red jerseys still dot the stands at Nationals Park even as they step out of the visiting dugout wearing different colors. Nor is it to re-engineer the contract negotiations that left Desmond turning down a proposal from the Nats before he became a free agent following the 2015 season. He left first for Texas in 2016 and eventually the five-year, $70 million contract that has him part of a Colorado team that has made back-to-back postseason appearances.

He is not as central to that effort as Charlie Blackmon or Nolan Arenado, but the guy who is essentially the last Expo — drafted out of high school in 2004, when the Nats were still in Montreal — he has seen enough to know how this works. That guy in the locker next to you? He might not be there in two years or two months or two days. Shoot, the locker next to Desmond’s in the visitors’ clubhouse belongs to none other than Daniel Murphy, the hitting savant of a second baseman who won two division titles with the Nats — after Desmond was gone.

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Nothing’s permanent.

“You have to enjoy your time together,” Desmond said. “If you guys have stuff that you want to accomplish, consider the window.”

The window, to be clear, involves specific players, not an entire franchise. The Nationals have intended to compete every season since the 2012 campaign Desmond remembers so fondly. They entered the four-game series with the Rockies holding the top wild-card spot, and should they secure a berth this October, it will mean they haven’t missed the playoffs in consecutive seasons since 2010 and 2011. The New York Yankees can’t say that. The Boston Red Sox can’t say that. The Chicago Cubs and St. Louis Cardinals can’t say that, either. For all their flaws and foibles this season, that’s still remarkable.

But the relative consistency comes with a changing cast of characters. Ian Desmond stepping into the Nationals Park batter’s box in a Rockies uniform isn’t so much jarring as it is a reminder that this is all fleeting. The only players who suited up for the Nats in 2012 and appear here now are Strasburg, Zimmerman and catcher Kurt Suzuki, who was acquired in a midseason trade back then and returned just this offseason as a free agent.

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That turnover, even as it’s nearly complete, is also completely normal. The Los Angeles Dodgers have won every NL West title since 2013. The only players on that first team who remain now are lefties Clayton Kershaw and Hyun-Jin Ryu and closer Kenley Jansen. Different franchise, opposite coast, same revolving door.

The core of any team is always shifting. In 2013, the Nats added Anthony Rendon as a foundational piece of division winners in 2014, 2016 and 2017. And now no question is more central to the Nats’ core going forward than Rendon: A free agent at season’s end, will he be part of the team’s future?

The Rockies, by contrast, have no such question about their own third baseman because Arenado, also due to hit free agency this offseason, signed an eight-year, $260 million deal before Opening Day to stay in Colorado.

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That kind of move doesn’t just make one guy rich. It gives the clubhouse some unity. They did it for Arenado. They did it, on a smaller scale (six years, $108 million) for Blackmon, a four-time all-star. Might they do it for, say, shortstop Trevor Story?

“It solidifies the feeling like, ‘Look, we have a phenomenal owner who’s willing to spend money to keep players that he likes that add something to the organization,’ ” Desmond said. “That gives guys like Story hope that he can stay.”

With more than two months to go in the season, these Nationals already have used 43 players — and, lord willing, will add a 44th and maybe 45th before next week’s trade deadline. (Nothing personal against the current relievers, of course.) How many current position players appeared in even 50 games for the most recent division title winners, back in 2017? Just Rendon, Zimmerman, Trea Turner and Howie Kendrick. Only Max Scherzer and Strasburg remain as rotation stalwarts. Only Sean Doolittle and Matt Grace are regulars from the bullpen.

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The core of this team? It has to be Scherzer, Strasburg, Doolittle, Turner, Rendon and young outfielders Juan Soto and Victor Robles. The crowd that gathered to begin an important homestand has attachments, to varying degrees, to each of them. They are tenuous holds. If you like this team, value it. It not only will be different next year, it will be different next month.

What doesn’t change: the people. Rob McDonald is the only director of team travel the Nats have ever had. Perhaps no one knows each player from each team better. When McDonald’s wife gave birth to the couple’s third child — their first son — this month, he texted the name of the newborn to the former Nats shortstop: Desmond Brown McDonald. Not totally in honor of Ian. But it didn’t hurt.

“So awesome,” Desmond said Tuesday. “It’s still starting to sink in.”

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That’s the connective tissue, back to 2012, back to another Nats era. That’s why the crowd stood and cheered Desmond, the person, Tuesday night. Cores change, and that’s inevitable. The reminders are everywhere, all across the league: There’s no promises that this group is going to stick together.