Spurred by their hard-charging manager and soaring toward the postseason, the Washington Nationals are not mixing mercy with their baseball. As they clobbered the Chicago Cubs again Thursday night, the Nationals rejected etiquette. They took every extra base they could. They swung for the fences. They pummeled the Cubs until, overmatched on the field, the Cubs picked a fight.

The Nationals sealed a four-game sweep at Nationals Park with a 9-2 onslaught, a decisive victory that dropped their magic number to win the National League East to 18. But no one Friday morning will be talking about Bryce Harper’s ferocious base running, Adam LaRoche’s sixth homer in six games or Jordan Zimmermann’s rebound start. They will talk about the benches-clearing, bullpen-emptying, finger-pointing fracas.

The bad blood culminated in the sixth inning, when Cubs reliever Lendy Castillo fired a fastball at Harper’s waist. Harper ducked out of the way and took two steps at Castillo. Both rosters met in the middle of the diamond. Three players, including Nationals reliever Michael Gonzalez, would be ejected. Michael Morse and Edwin Jackson grew particularly animated.

The seeds had been sown for days by the combustible combination of a ticked-off team and a manager, Davey Johnson, who gives no quarter.

“If they get mad at my guys in the fifth inning swinging 3-0 or running,” Johnson said, “they better get used to it.”

All series, the Nationals had battered the Cubs as soundly as one major league team can trounce another. Wednesday night, Chicago Manager Dale Sveum called their meeting “men against boys.” By the end of Thursday night, the Nationals would outscore the Cubs 31-9 over four games. The Cubs lost their 86th game. Frustration filled their dugout.

“I think I’d be pretty pissed off if I was getting my teeth kicked in all [week], too, but you can’t lay down,” Harper said. “You’ve got to keep going, keep grinding, keep coming.”

In the fifth inning, the Nationals had already taken a 7-2 lead. For Johnson, it was not enough. He extols his players to not let the opponent dictate when to stop competing.Take nothing for granted. The Nationals, remember, lost a game to the Atlanta Braves this season after leading by nine runs. Many of them mentioned the game afterward.

Both Ian Desmond and Danny Espinosa stole second base in fifth. With a 3-0 count, Jayson Werth took a mighty hack at a 3-0 curveball. Some in the Cubs’ dugout considered that piling on. Johnson, in absolute terms, does not. He considers it baseball.

“Here we are in the fifth inning, we’re in a pennant race, we’re going to swing 3-0, we’re going to do everything,” Johnson said. “We ain’t stopping trying to score runs. Certainly, a five-run lead at that time is nothing. I think it was the bench coach’s frustration in us handing it to them for a couple days. If they want to quit competing and forfeit, then fine. But we’re going to keep competing.”

After Werth’s swing, Cubs catcher Steve Clevenger walked to the visitors’ dugout to change his catcher’s mitt. Cubs bench coach Jamie Quirk started yelling at Nationals third base coach Bo Porter about the steals and the swing. Porter, who played safety for four years on the University of Iowa football team, paced toward Quirk, yelling and pointing.

“The fracas was started because all that stuff that happened that was instigated by Quirk screaming out at Porter,” umpiring crew chief Jerry Layne said. “And the obscenities that he screamed out, I just felt was inappropriate and that’s what caused everything.”

Said Johnson: “It was getting real personal and it’s [Porter] sticking up for me because it’s really my decision on what we do on the field.”

Porter did not comment directly about his exchange with Quirk, but he defended the Nationals’ style.

“Every time you start the game, there’s two teams out there,” Porter said. “And when I was younger, I did Gold Glove boxing. My trainer would always tell me before the bell rung, he said, ‘Just in case you didn’t know, when this bell ring, that guy over there, he’s gonna hit back.’ ”

The Nationals poured out of their dugout and across the field, but Quirk and Porter were separated before anything could break out. Quirk was ejected.

The top of the sixth played out without incident. Harper, a magnet for the middle of things, led off the bottom of the sixth. Castillo rifled a 96-mph fastball at Harper’s belt buckle. There was no mistaking the intent.

“If you come into our house and try to mess with our kid brother, that’s how we look at it,” Gonzalez said. “You’re not just going to come in and please as you do with that.”

Harper stalked toward the mound. Players streamed from the bullpens and benches. Werth and Ryan Zimmerman made a beeline for Harper to restrain him, and he settled around the plate.

“I wasn’t going to do anything stupid,” Harper said. “I’m just trying to check off everything on my list. I’ve gotten thrown at, gotten hit, stole home, we’re winning, homers. Everything that’s going on, just checking it off the list. That’s one of them. Just trying to stay calm in that situation and not do anything stupid.”

Nationals and Cubs coaches tried to separate the players. No punches and a lot of yelling. A typical baseball dust-up — at first.

As the relievers retreated to their bullpens, Cubs first baseman Anthony Rizzo put his hand on Gonzalez’s shoulder, which led to Gonzalez and a Cub screaming at one another. The peace had been broken. The teams charged at one another again.

Sean Burnett found himself in the middle of the scrum. Clevenger, who would be ejected, shoved Morse and touched off brief chaos. Jackson screamed and pointed. An umpire tumbled to the ground as he pulled Desmond from the pile.

“Just tensions were high a little bit,” Morse said. “Both teams were a little mad. But it was under control. The umpires did a great job. Nothing came about it. And we kept playing. So it was good.”

Johnson held back Morse and bench coach Randy Knorr held back Jackson. (Afterward, Jackson declined to comment.) Johnson was protective of his own players — they had everything to lose, and they were feuding with a team with nothing to lose.

“I was proud of my guys for not really overreacting and taking a chance,” Johnson said. “We’re in a pennant race. We’ve got to be mild citizens. We don’t start things but we’re not going to completely back down from anything.”