The Nationals were on the verge of adding to a storybook season in which they produced their first winning record since moving from Montreal seven years ago, but the Cardinals staged a repeat of last year's comeback dramatics and kept alive their hopes for a second straight World Series title.
The five-game St. Louis triumph in the division series, climaxed by the four-run, ninth-inning rally, set up a battle with San Francisco for the National League pennant. It will be a meeting of the last two World Series champions and it will begin in San Francisco on Sunday.
Washington jumped to a 6-0 lead through three innings and spent the rest of the night trying to fight off the Cardinals.
St. Louis closed to within 6-5 in the eighth on a homer by Descalso, but the Nationals added an insurance run in the bottom half of that inning and Drew Storen came on for Washington in the ninth hoping to touch off a rare baseball celebration in the nation's capital.
Carlos Beltran opened the ninth with a double, but Storen retired the next two batters to move the Nationals within one out of a victory.
That brought up Descalso, whose hard grounder up the middle bounced off the glove of a diving Ian Desmond at shortstop and trickled into center field. Beltran and pinch-runner Adron Chambers scored to create a 7-7 tie.
Descalso stole second while Kozma was at the plate and Kozma's base hit to right produced two more runs and gave the Cardinals the lead.
St. Louis reliever Jason Motte made no mistakes in the bottom of the ninth, retiring the side to finish off two innings of work and gain credit for the dramatic victory.
The game was strikingly similar to the 10-9 St. Louis victory over Texas in Game 6 of last year's World Series, when the Rangers twice were within one strike of finishing off the Cardinals. But St. Louis rallied to tie the game in the ninth inning and rallied again to win it in the 10th. A victory the next night in Game 7 gave the Cardinals the championship.
St. Louis demonstrated its tenacity, however, scraping out runs along the way on a wild pitch, a bases-loaded walk and a ground out.