While Nationals starter Dan Haren was being lit up in his first start with Washington, Reds starter Homer Bailey, above, was cruising through six shutout innings, allowing just two hits and two walks while fanning six. (Jamie Sabau/Getty Images)

The conditions conspired against Dan Haren on Friday night in his introduction to life as a Washington National. He walked to the mound at the center of the coziest ballpark in the majors, against one of the league’s most fearsome lineups, nine days after he last pitched. The setting promised to challenge Haren. It did not threaten to overwhelm him — but yet it led to unmitigated disaster.

In his Washington debut, Haren yielded four home runs in a four-inning calamity as the Nationals suffered their first loss of the young season, a 15-0 shellacking at the hands of the Cincinnati Reds. After their season-opening sweep over the lowly Miami Marlins, the Nationals came to Great American Ball Park and started a series matching playoff contenders with their most lopsided loss since baseball returned to Washington in 2005.

“I’ve had my share of bad games along with good games,” Haren said. “They’re tough to deal with. But the sooner you forget about it, the better. It’s still a long season. I’ve probably 30-plus more starts left. This start isn’t going to define how this year’s going to go for me.”

For one night, the Nationals’ swagger disintegrated. While Haren spent the majority of his night turning around and looking up, Reds right-hander Homer Bailey dealt for six two-hit innings. The Nationals managed only five hits while striking out seven times. Relievers Zach Duke and Henry Rodriguez turned a rough night at the park into a dumpster fire, the damage culminating with Xavier Paul’s pinch-hit grand slam off Rodriguez to cap a seven-run seventh.

“You just dismiss these,” said first baseman Adam LaRoche, who exited the game with back stiffness. “These are going to happen. I have a feeling we’re going to be on the better side of a lot of these games this year.”

The Washington Post’s Mike Wise offers his extra points about Bryce Harper and his future professional baseball career. (Post Sports Live)

The end turned the final score an eyesore, but the Nationals remained in a tie for the best record in the majors and still had 158 games left to even out their run differential. There was probably only one conclusion that mattered, and it was obvious: Haren, for reasons that extended beyond Friday night, needs to cut down on allowing homers.

The Nationals signed Haren, 32, in December to a one-year, $13 million deal to anchor the back of their rotation. The team’s first three starters allowed one run over 19 innings. In a different stadium against a very different opponent, Haren allowed six runs on nine hits in four innings. He struck out five and walked none, which offered little solace.

“I know I’m better than that,” Haren said. “There’s no use dwelling on it. It’s over with.”

Still, Haren searched for a way to improve. Afterward, he sat at his locker and chatted with catcher Kurt Suzuki. They decided he needed to throw more four-seam fastballs, because his two-seam fastball was running back over the plate too much.

“I know Dan,” Suzuki said. “It’s not him. It’s not who he is. It’s not the type of pitcher he is. He’ll get better.”

The small confines of Great American Ball Park and the sample size of one start made on nine days of rest suggests Haren’s beginning is not necessarily a harbinger, especially since three of the four homers landed in the front row and two were argued. But, given Haren’s homer-laden spring training, the trend is concerning.

In 2011, Haren allowed 0.76 home runs per nine innings, which ranked him a respectable 29th in the major leagues. In 2012, Haren’s home run rate jumped to 1.43 per nine innings, more than all but seven big league starters. Haren turned fell from his perch as an ace, in large part, because he started giving up far more homers.

The beginning actually portended good things for Haren. He fired his fastball 90-91 miles per hour, threw a darting cutter and induced four swing-and-misses from the first five batters he faced. Just as he settled into a rhythm, his start careened off track.

With one out in the second, Todd Frazier clobbered a first-pitch, 90-mph fastball into the upper deck hanging above the left field fence. Frazier had barely finished putting his helmet back in the rack when Zack Cozart pummeled his very next pitch, another fastball, to left field. The drive snuck into the first row, possibly caught by a fan leaning over the fence.

“I honestly wasn’t 100 percent sure,” center fielder Denard Span said. “Off my first thought, I thought it was a home run. Bryce [Harper] kind of said the same thing. I wasn’t 100 percent sure.”

Johnson trudged to argue for fan interference. According to Johnson, home plate umpire Jordan Baker told Johnson he needed to ask second base umpire Jerry Meals about the ruling, since Meals made the original home run call.

“I mean, I got to walk out there?” Johnson said. “I’m telling you, check it. I didn’t get any response from the left fielder or the center fielder. So I didn’t push it. But MLB should check that. ”

The Nationals trailed only 2-0 with nearly a full game left. In the third, Cozart landed the most damaging blow. With Joey Votto and Brandon Phillips on after singles, the Reds shortstop crunched an 0-1 cutter into the first row. His second homer traveled 348 feet, a double in most parks, but here it flew far enough to give the Reds a 5-0 lead.

Shin-Soo Choo, the Reds’ new leadoff hitter, delivered the final salvo. With one out in the fourth, Choo smashed a 2-1 cutter to center field. After the ball landed in the lap of a fan leaning over the fence, right fielder Jayson Werth signaled the dugout to call for a review. This time, the umps checked – and ruled the homer would stand.

“When stuff like this happens, it seems like you’re handcuffed out there,” Suzuki said. “Nothing can go right. They break bats, it’s a hit. Mistake, it’s a hit. There’s some games you get away with some mistakes. Tonight wasn’t one of those. I felt pretty helpless out there.”

The night tumbled into absurdity. In the seventh inning, a fan jogged from the left field seats to the infield, slid into second base and high-fived backup Reds catcher Devin Mesoraco. The Reds kept pounding the ball until an RBI double off Ryan Mattheus provided the final, historic margin. The Nationals’ previous worst loss came June 17, 2009, when Jason Simontacchi started in a 15-1 loss to the Detroit Tigers.

“We got out you-know-what kicked,” Johnson said. “But there’s always tomorrow.”