Nationals shortstop Wilmer Difo can’t make the play on a groundball in the fifth inning against the Cardinals. (John McDonnell/The Washington Post)

Given the Washington Nationals’ recent propensity for scoring runs, a base running mistake in the first inning Wednesday afternoon seemed unlikely to be decisive. With their lineup, its long at-bats, its ability to punish misplaced pitches, its bottom-of-the-order pop, the Nationals seemed likely to get another chance.

But the problem with relying on offense, as the Nationals discovered in their 6-1 loss to the St. Louis Cardinals at Nationals Park, is that bats are unreliable tools, offense too flighty and fickle to overcome repeated mistakes. So as the Nationals’ bats went silent Wednesday, a first-inning mistake grew in its importance and loomed as a glaring reason among many that the Nationals fell to 5-4.

The telling mistake came after Anthony Rendon singled, pushing Adam Eaton to third and creating yet another Nationals scoring chance in a series full of them. With Rendon on first, Cardinals pitcher Mike Leake tossed a routine keep-him-close throw to first.

Instead of diving back, which might have helped him dodge the tag — or at least served him better for appearances’ sake — Rendon spun back to the base upright. Replay revealed Matt Carpenter had tagged him out.

“I just got back,” Rendon said. “I don’t know. I just got back.”

Instead of two on with no out for Bryce Harper, the Nationals had one on and one out — seemingly a small difference for the most productive lineup in the majors through the season’s first week. But unlike Monday or Tuesday, when they bludgeoned mistakes into irrelevance, when they created scoring chance after scoring chance, the Nationals did not bounce back. Leake retired 19 straight Nationals after that play, beginning with a strikeout of Harper.

“We had him on the ropes in that first inning and then the pickoff and the strikeout to Bryce,” Nationals Manager Dusty Baker said. “It would’ve been a whole lot easier with nobody out in that situation.”

By the sixth, the Nationals had not mustered another base runner. By the time Max Scherzer finished six innings and allowed one earned run, the Nationals trailed 3-0. For the first two games of the series, like most of the first week of the season, the Nationals hit themselves out of trouble. But when the margin for error was smaller, mistakes loomed large.

The Nationals made two errors Wednesday. One came leading off the fifth, when shortstop Wilmer Difo bobbled Eric Fryer’s grounder to his backhand. That runner scored, as did another that inning, both of them unearned. Rendon made a throwing error a few innings later, though Shawn Kelley pitched around that one.

The Nationals made seven errors in three games against the Cardinals and allowed five unearned runs. In nine games, they have made nine errors. Last year, they made their ninth error in Game 27.

“It always concerns me when you don’t catch it or throw it because you’re giving away outs,” Baker said. “You’re giving away outs, and the game wasn’t designed for 30 outs versus 27. We just got to tighten our defense and tighten our entire game up.”

Though not as egregious as errors, walks continue to trip up these Nationals, too. Scherzer, for example, got the loss Wednesday despite striking out 10 batters for the 50th time in his career. Among active pitchers, only Clayton Kershaw has more, with 51 such games.

But Scherzer walked two batters and hit another. Two of those three batters scored. Sammy Solis walked a batter in the ninth. He eventually scored, too. Leadoff walks troubled the Nationals as their bullpen lurched through three games in Philadelphia. Nationals relievers have walked nine batters in 29⅓ innings.

They also have allowed nine home runs, an epidemic that continued into Wednesday when Joe Blanton allowed a three-run shot to Stephen Piscotty in the ninth. Walks and home runs constitute capital crimes for relievers, who generally feel they can control two things: throwing strikes and keeping the ball in the ballpark.

“We’re just making a few bad pitches, and we’re getting penalized with it,” Nationals catcher Matt Wieters said. “. . . I think we get them back to locating their fastball, and then throwing their breaking stuff like it is they’ll be fine. Right now, it seems like every time when they leave one in the middle somebody puts a good swing on it.”

Wieters shouldered the blame for the three wild pitches credited to Scherzer on Wednesday, one of which allowed a runner to get into scoring position. Scherzer threw two wild pitches all of last season.

Mistakes on the bases, in the field, or behind the plate have not totally doomed the Nationals, who had a chance to sweep a perennial contender all the same. Until Blanton surrendered the home run to Piscotty in the ninth, the Nationals had chances to win. They rallied to bring Harper to the plate as the potential go-ahead run with two on and two out in the eighth. Harper lined out to third, a few feet away from a game-tying double.

Such is the trouble with relying on offense, which is notoriously unreliable, even for lineups like this one. Perfect swings do not always inflict damage. Mistakes — like the two unearned runs the Nationals allowed in a game they trailed by two entering the ninth Wednesday — almost always do.