Bryce Harper’s home run does not prevent Nationals from getting last laugh at Citizens Bank Park | Bob Brookover

The party atmosphere that has returned to Citizens Bank Park was in full swing through the first six innings Tuesday night. It did not matter that it was a school night in early April. The Phillies were in town and a crowd of 38,073 wanted to see if they could keep the early-season momentum going against Bryce Harper’s former team, the Washington Nationals.

Harper had them dancing from the chandeliers in the bottom of the third inning when he won a classic confrontation with former teammate Stephen Strasburg by blasting a three-run homer into the left-field seats. Strasburg tried to throw a third straight changeup by Harper, but the Phillies’ right fielder stayed back and hit his fourth home run of the season.

By the end of four, it was 6-1, thanks to a two-run triple by Jean Segura and the joint was jumping.

Better still, Aaron Nola, after a rocky three-inning start last week at Nationals Park, looked like a staff ace again. He had allowed a couple of runs through the first six innings, but one of them was the result of a dropped fly ball that Harper should have caught in right field.

He might not have looked entirely like the Cy Young Award candidate he was a year ago, but it was a step in the right direction and there was no reason to believe this night would end in a coyote-ugly 10-6 loss in 10 innings.

“It was obviously a tough loss,” manager Gabe Kapler said.

By the time it was over, the ballpark had mostly emptied and there was a serious chill in the air both literally and figuratively. Those who remained were unhappy with the home team and the umpiring crew for ruling that Juan Soto’s go-ahead three-run was fair.

A scoreboard replay indicated it might have been foul, but replay officials in North Jersey indicated otherwise and Harper confirmed that view.

“I thought it was fair,” Harper said. “It looked like it went right over the foul pole, so I thought it was a fair ball. As a team, I didn’t think we finished.”

They should have been most disturbed by the state of the Phillies’ bullpen, which was short at this early stage of the season.

The drastic fall-from-ahead defeat was set in motion by a Maikel Franco throwing error to start the seventh inning. When Yan Gomes followed with a home run, the Phillies lead was down to two and the party was over.

Two batters later, former Phillie Howie Kendrick jumped on a hanging curveball from Nola for the Nationals’ second home run of the inning off the Phillies’ ace and their fifth in two games this season.

“I felt better than I did in D.C., but it’s frustrating,” Nola said. “The guys scored six runs and I put five on the board. They gave me a good lead and it’s frustrating when the other team comes back.”

Nola’s lowered his from 7.00 to 6.46, but that’s still a lot more elevated than he’d like.

“Nobody wants outing like this,” he said. “I guess you can say it’s a slow start, but I’m going to keep battling and competing. That’s all I can do.”

Even more disturbing than Nola’s seventh-inning meltdown was the bullpen collapse. Kapler knew going in that he was shorthanded because Adam Morgan, Pat Neshek and David Robertson were unavailable. They had all pitched three times in the last four days.

It would be fair to question how Kapler handled his shorthanded pen.

He only asked for two outs from Seranthony Dominguez and got them in the seventh. Hector Neris held the Phillies’ lead at one with a scoreless 30-pitch eighth inning during which he threw 26 splitters.

Edubray Ramos was asked to get the final three outs, but could only get two before Victor Robles planted a slider in the left-field seats to tie the score at 6-6.

In this case, two out of three was bad. Really bad. And it would become a disaster in the top of the 10th inning when the Nationals scored four times off Jose Alvarez.

What looked like a sure 8-2 start for the Phillies had dissolved into a devastating defeat.