Perhaps no team in MLS has relied more on its defense than D.C. United. With an injury-hampered attack that has scored the third-fewest goals in the league, United rode to fifth place in the Eastern Conference standings on the back of two sturdy goalkeepers and a rotating wall of dependable defenders.

That wall collapsed against the Philadelphia Union on Saturday night. A pair of sloppy penalties put United in a two-goal halftime hole, and a third goal two minutes after intermission put the visitors out of their misery before 18,463 at Talen Energy Stadium.

Saturday’s 3-0 result marked United’s eighth shutout loss of the season. The Union (8-6-5) snapped a two-game skid and climbed within one point of first-place New York City FC in the Eastern Conference standings.

“We were just late to show up to this match in every aspect,” said United Coach Ben Olsen, whose team fell to 5-7-6. “They had more everything in that first half. We give up a set piece to make it 3-0, and it’s over.”

It was hardly the performance Olsen expected from a defense that is fully healthy for the first time this season. With goalkeeper Bill Hamid looking sharper than ever and a deep back line that Olsen recently dubbed “a luxury of riches,” United’s vulnerabilities appeared to lie entirely on the other end of the field entering the season’s midway point.

Steve Birnbaum, who suffered a minor injury after returning from Copa America duties earlier this month, got his first start at center back since United’s previous trip to Chester on May 20. He replaced Bobby Boswell , who handed the captain’s armband to right back Sean Franklin.

Kofi Opare partnered alongside Birnbaum in central defense. Perhaps United’s most reliable center back in recent weeks, Opare suffered his worst outing of the season against the Union, and D.C. paid the price.

Opare committed two fouls on Union striker C.J. Sapong in dangerous areas inside the game’s first eight minutes. His first major mistake, though, came when former United mainstay Chris Pontius took off on a breakaway in the 19th minute.

Pontius dribbled down the middle with a head of steam, then found himself one-on-one with Opare just inside the penalty area. A sharp cut left prompted Opare to stab at his former teammate’s feet, bringing him to the turf for a penalty and drawing a yellow card in the process.

Union midfielder Roland Alberg hesitated on the ensuing spot kick to get Hamid diving right, then calmly slotted his shot the other way to put Philadelphia ahead.

Marcelo Sarvas issued an equally clumsy challenge 10 minutes from halftime. Ilsinho plunged into the penalty area and took a touch past Sarvas, who lunged into the Union midfielder with his shoulder. Ilsinho drove the penalty kick low and left, just beyond Hamid’s reach.

Sapong, back in the Union’s starting lineup after missing three games with an injury, wreaked havoc in the final third all evening with darting runs and quality hold-up play. He was the beneficiary of another United foul just outside the box 34 seconds into the second half. Alberg floated the ensuing free kick to the back post, where Sapong headed it toward an unmarked Ilsinho near the edge of the six-yard box. Ilsinho ripped his one-timer past Hamid to all but secure victory.

“For me, I thought C.J. was their best player in the first half,” Franklin said. “He’s battling. He’s strong. He’s flicking balls. He’s running into the channels. When you play against a guy like that, it’s tough to defend.”

Opare picked up his second yellow card with a reckless challenge three yards outside the box. United played the final 30 minutes with 10 men.

“It’s just not sharp from us. We’ve got to do a better job,” Franklin said. “Tonight they just had better energy. They wanted it more. This is a game where you need to look at yourself in the mirror and figure out what you can do to get yourself better.”

Alhaji Kamara got his first MLS start but didn’t offer much in 57 minutes of action. Alvaro Saborio replaced him in the second half and was similarly ineffective.

“This isn’t the end of the world,” Olsen said. “It’s a loss away at Philly, a very good team, particularly when C.J. comes back and they play with this type of energy. It’s a tough place to go.”