Atlanta's Tommy Hanson extended his scoreless innings streak to 26 before Dunn's 300th career homer and the Washington Nationals rallied for four eighth-inning runs and a 5-3 victory over the Braves on Saturday.
"That 300th home run would have been hard to talk about. It would have been a useless home run if we hadn't won the game," said Dunn, whose RBI single off Eric O'Flaherty snapped a 3-all tie.
Hanson departed with a 3-1 lead after seven innings and was in line for his fifth straight victory before the Braves bullpen blew up, halting Atlanta's winning streak at a season-high five.
"It was remarkable," said Nationals manager Manny Acta.
Mike Gonzalez (3-1) loaded the bases with one down in the eighth on a pinch-hit single by Ronnie Belliard and a pair of walks before Peter Moylan relieved and yielded a two-run, game-tying single to Ryan Zimmerman.
O'Flaherty replaced Moylan and was greeted by Dunn's decisive single up the middle on the first pitch he threw. Josh Willingham followed with an RBI single to left.
"That's the most frustrating part right now. I could really care less about how I did. Tommy goes out there and does such a great job, and obviously our offense is doing really well," Gonzalez said. "We're playing really good ball. Hopefully this doesn't put a chink in what we're doing right now."
Atlanta's bullpen had pitched to a 3.98 ERA and had retired 167 of 243 first batters faced (69 percent) before Saturday's implosion.
"It's been terrific. I don't think we're ever due [for a bad game], but it happens. You're not going to be perfect all the time," Braves manager Bobby Cox said.
Lannan was the beneficiary of the rally. The left-hander allowed three runs and nine hits in eight innings, ending Washington's losing streak at four games. Lannan (6-5) walked one and struck out one.
Mike MacDougal worked the ninth for his fourth save, escaping a two-on, one-out jam.
Hanson, who was recalled from Triple-A Gwinnett on June 7 to make his major league debut, took a no-decision in that game and has yet to lose. He allowed only Nick Johnson's single to center in the first and a second-inning infield hit by Anderson Hernandez over the first six innings.
The right-hander walked one and struck out five, lowering his ERA to 2.25 in a 105-pitch effort.
Acta said Hanson deviated from the video scouting report the Nationals had on him, not hurting himself with walks and failing to fall into pitch patterns hitters could recognize.
When asked what was working for him, Hanson said: "I think everything. I felt good with all my pitches and my command of everything. I was just trying to keep them off balance and it worked out."
Dunn hit his milestone blast leading off the seventh, crushing a 2-2 fastball deep into the second deck in right field to ruin Hanson's shutout bid. The 29-year-old is the fifth player in major league history to reach that total in less than 4,200 at-bats, succeeding in his 4,145th at-bat. The others are Babe Ruth (3,830), Mark McGwire (3,837), Ralph Kiner (3,883) and Harmon Killebrew (3,928).
The Braves took a 1-0 lead in the third on Nate McLouth's RBI single. Brooks Conrad led off with a single to center, moved to second on Hanson's sacrifice bunt and scored when McLouth blooped a single over second base.
- Lannan has thrown six straight quality starts.
- Atlanta and Washington are tied for the National League lead with nine victories from rookie starting pitchers.
- Washington is 3-2 on July 4 since moving from Montreal before the 2005 season, winning three straight on Independence Day.
- Braves OF Matt Diaz is 11 for 17 (.647) with five RBI in his past five games.
- Dunn's 300th home run ball was caught by U.S. Army Col. William Sanders, who returned it to Dunn, asking for nothing in return. Dunn gave him an autographed jersey, a couple of autographed baseballs and told Sanders to call him if he ever wanted tickets to a game.