CARY, N.C. -- Four years ago, circumstances made them interchangeable.
For Crystal Dunn and Julie Ertz, each in her own way, that was the first step toward becoming indispensable.
On the eve of the 2014 CONCACAF Women's Championship, Dunn suffered a knee injury. In camp as an extra body for training, Ertz took her place on the U.S. roster.
The 20th player on a roster of the same number, Ertz didn't play a minute in the five wins that qualified the U.S. for the World Cup the following year. But disappointment at missing out on the initial roster became an opportunity to get her foot in the door. That opportunity became a World Cup starting assignment. And that World Cup transformed her life almost overnight.
For Dunn, meanwhile, opportunity evaporated into disappointment, first because of injury and then missing the World Cup roster the next year.
Two roads diverged in a moment four years ago in Kansas City.
After both players started a World Cup qualifier for the first time in a win against Mexico on Thursday, those roads may yet converge in France next year.
"It just shows you that life goes on," Dunn said this week. "You have a disappointment and you have to brush yourself off and get on with it. I don't regret anything."
Though she might prefer if the symmetry was little less on the nose, Dunn is back where she was four years ago with the national team before she missed out on qualifying because of the injury. An attacking player in almost every other setting since late in her college career at North Carolina, she was a contender for minutes at an unsettled outside back position in 2014 and has again settled in as the de facto starter at left back for the U.S. over the past seven months.
"It's a combination of qualities and need," U.S. coach Jill Ellis said of playing Dunn as a defender. "I think Crystal has just a phenomenal skill set in terms of her ability and desire to go [one-on-one], so it kind of naturally makes her a wide player for us. I think we don't have a whole lot of depth in our outside back positions, so when we put Crystal there [against England in the SheBelieves Cup], I thought she did a fantastic job.
"I think the way we can kind of manipulate the game and get her as high as possible into the final third as much as possible, that just helps us in our attacking game."
That was evident in the win against Mexico. On one of many charging runs down the left flank, Dunn gathered herself and delivered a cross that a wide-open Tobin Heath headed for a goal.
"She's playing as a left back, but she actually comes in and almost plays as a No. 10 and then almost plays as a winger, as well," Heath said. "It's incredible to have a player like that on the team that is so versatile, but not even [just] versatile -- she's very good in every single position on the field."
A lot of work went into that. Dunn didn't go into hibernation after missing out on the last World Cup. To the contrary, few players were busier or accomplished more in the past four years. She won NWSL MVP in Washington. She signed with one of the world's biggest clubs, Chelsea, and played in the Champions League. She returned stateside as the centerpiece of a blockbuster trade and helped the North Carolina Courage win a title in the most dominant season in NWSL history. She got back into the national team and played in the Olympics.
She doesn't hide her positional preference, makes no excuses for thinking of herself as the attacking player who had such success with Washington and North Carolina. But this is one more challenge, the list of them in the past four years longer than her almost perpetual smile lets on. Behind that pleasantness is a long memory of the path followed to this point.
"I just think you can't focus on what happened four years ago, what could have happened." Crystal Dunn
"I'm only really frustrated when I feel like I can't be at my best in a new position," Dunn said. "I think that's when I get extra motivated to keep getting better. I can't really sit and pout and be like, 'Oh, I'm playing outside back and I don't feel as confident as I would if I was in an attacking role.' Now I'm motivated to be the best outside back I can be."
Rewind four years and it was initially Ertz left to wonder what twists and turns awaited in her path back to the national team. A decorated player in the youth national system and the NWSL rookie of the year in the season that had just ended, she had yet to make an impression with the senior team when left off the qualifying roster. Her rapid ascent appeared to have finally stalled.
"Disappointment with myself that I didn't do enough to make the initial roster," Ertz recalled of the emotions swirling in that moment. "I was grateful for an opportunity that as an alternate I could go in and kind of get the experience from the outside looking in. And I think a third one would be the motivation factor from it. I learned a lot -- obviously not taking anything for granted. But it motivated be to show how bad I wanted it.
"I knew I wanted it, I knew I wanted to be a part of the team."
Less than eight months later she was starting in the World Cup. She played every minute as a center back and was named one of the tournament's best performers. Perhaps indispensable.
"I didn't really have any expectations," Ertz said of the 2015 World Cup. "I just went in there wanting to do my best and continue to grow on and off the field. Things kind of catapulted so fast ... that I didn't have time to reflect on that necessarily. And I was enjoying the time so much, seeing hard work pay off, I just wanted to continue that."
That year also established that the U.S. needed her as much as she needed it. Ertz plays with an energy that radiates, energy that a team with ample talent but fewer domineering personalities needs. She may not be the conductor, but she is the metronome. So even as she changed positions last year, shifting to the defensive midfield role that is more natural to her, it was a matter of where she best fit as a known commodity.
Now she is a cornerstone, a multi-talented player with the skills and experience to slide into the defensive line for extra cover when Dunn pushes forward out of the back line.
What if Caesar hadn't crossed the Rubicon? What if Germany's Celia Sasic hadn't missed a penalty against the U.S. in a World Cup semifinal four years ago in Montreal? What if Carli Lloyd had? There is no limit to the history that didn't happen, alternate endings perhaps less important than the reminder that such small moments often determine the history we know.
Perhaps Dunn and Ertz were destined to make their marks no matter what paths they took.
Or perhaps that single roster move for a 2014 tournament changed everything for 2019.
"No one knows what could have happened if I did make that roster for qualifiers and if I was able to play some games," Dunn said. "I wasn't the player that I am at this point in my life. I just think you can't focus on what happened four years ago, what could have happened.
"That was a point in my career when I was young, I was getting injured a lot. And now I'm a different person, different player."
What is clear is that increasingly makes Dunn, like Ertz, a player the U.S. can't do without.