The Nationals became the first baseball team from Washington to earn a post-season berth in 79 years.
After suffering through nothing but losing seasons since moving from Montreal in 2005, the Nationals have become the sport's surprise team of the year -- winning for the 91st time and wrapping up at least a wild card berth.
Washington moved 5 1/2 games in front of Atlanta in its bid to capture the NL East title.
The franchise was created in 1969 as the Montreal Expos and the only time the Expos made it to the playoffs was in 1981. The Washington Senators won the American League crown in 1933 before losing to the New York Giants in the World Series.
The Nationals became the second team to earn a post-season spot Thursday -- the Cincinnati Reds having clinched at least a wild card spot with its afternoon victory over the Chicago Cubs.
Detwiler (10-6) allowed a run on three hits with five strikeouts in six innings to get the win. The bull pen gave up one more hit and Drew Storen struck out the side in the ninth to record his third save.
Ryan Zimmerman put Washington in front with an RBI double in the third and he scored later in the inning on a wild pitch. Danny Espinosa also doubled home a run in the fourth and he soon scored on a sacrifice fly by Kurt Suzuki.
The loss went to Chris Capuano (11-11), who allowed all four runs on six hits in five innings.
Los Angeles fell three games behind St. Louis for the second wild card berth in the National League.