WASHINGTON — Pete Alonso took responsibility for one eighth-inning play that led to the Nationals scoring Sunday, but on another, he felt he handled the situation correctly.

There was the obvious: The Mets first baseman’s throw on a grounder pulled Francisco Lindor off second base and helped extend the inning for the Nationals, allowing them to score the go-ahead runs.

More subtly there was Alonso’s underhand throw to the plate on Lucius Fox’s safety squeeze earlier in the inning. Dee Strange-Gordon beat that throw for the tying run.

“It was a bang-bang play at home, and I thought I made the right decision,” Alonso said after the Mets’ 4-2 loss at Nationals Park that ended their three-game winning streak to start the season. “I thought I made the right play. If I were to throw it [overhand] I feel like it would be too quick and too close of a distance. I didn’t want to make a bad throw because the other runner is on base. I wanted to secure the baseball and make a good flip to Tomas [Nido]. I feel like I did that.

“Dee is one of the fastest guys in the league and he made a real good slide and that play offensively was executed perfectly. I thought I did a pretty good job on defense. It was just a bang-bang play and he was safe.”

In the middle of the meltdown (the Mets led 2-1 entering the inning) was manager Buck Showalter’s decision to use Trevor Williams over the more battle-tested arms in his bullpen. But Showalter was intent on getting Williams into a game while resisting the temptation of using Seth Lugo, Adam Ottavino or Drew Smith for a third time in the series. Trevor May was presumably the closer Sunday with Edwin Diaz on bereavement leave following his grandfather’s death.

Chasen Shreve, another reliever Showalter wanted to ensure got work, pitched 1 ¹/₃ innings scoreless before he was sent back to the mound for the eighth. Yadiel Hernandez singled leading off the inning. Williams entered and allowed a single to Maikel Franco that put runners on the corners.

“We’re too early in the season to be throwing guys three out of four days,” Showalter said. “We said the whole offseason with the lockout and everything, we are going to be careful. That is why we’re spreading it around.”

The doozy occurred after Fox’s safety squeeze had tied it 2-2, with one out and runners on first and second, Alonso fielded Cesar Hernandez’s grounder and fired to second. The throw was wide of the base and everybody was safe. Juan Soto followed with a grounder that Alonso fielded and threw to the plate, nailing Franco. But Nelson Cruz’s ensuing two-run single was the backbreaker for the Mets.

“On the double-play ball, I just missed the throw,” Alonso said. “I don’t know why. My feet were set. I fielded the ground ball cleanly. There was no rush. There was no need to rush. I feel like I was calm, cool and collected. I just missed the throw and as a result that kind of put the team in a real bad hole.”

Williams called his outing a “good learning moment” on which to build.

“I felt like I’m capable when called upon to have a high-leverage spot,” he said. “At the end of the day we chalk it up as a loss, but it’s a learning moment and I am looking forward to getting out there.”

The Mets used a pop and patience to score twice in the fourth and take a 2-1 lead. The pop came from Lindor, who homered against Erick Fedde leading off the inning. But with two outs the Mets started another rally, which included a walk to Dominic Smith following Eduardo Escobar’s double. Mark Canha drove in the go-ahead run with a single off the diving Soto’s glove in right field.

Cruz’s two-out homer in the first inning was the Nationals’ only damage inflicted upon Carlos Carrasco, who allowed one run on two hits over 5 ²/₃ innings, departing after 74 pitches.

“It’s good to win the first series of the year,” Alonso said. “But dropping this one late kind of stinks.”