On 5 March 1940, Argentina beat Brazil 6-1 in Buenos Aires. It remains the largest margin of victory in the rivalry between the two countries.
The match was part of the Roca Cup, an irregular tournament played between Argentina and Brazil that dated back to 1913 when it was founded by General Julio Roca (pictured). Roca was a football enthusiast who also happened to be a former president of Argentina and, in 1913, was their ambassador to Brazil. He proposed the tournament to help develop the sport and also donated the trophy.
The 1940 edition was the fifth tournament, with Brazil and Argentina evenly splitting the previous four--Brazil won in 1914 and 1922, while Argentina won in 1923 and in 1939 (the 1939 tournament actually extended into February 1940). The 1940 tournament kicked off on 5 March with a table format. Three matches were scheduled, with the winners receiving two points for a win and one point for a draw.
Argentina opened the tournament (and defended their recently-won title) with a dominant 6-1 smackdown--the most lopsided victory by either side in the rivalry and Brazil's worst loss since their record 6-0 defeat to Uruguay in 1920. Brazil won the second match, 2-3, but Argentina again rolled in the final match, winning 5-1 to top the final table with four points to Brazil's two.
Despite the crushing losses, Brazil came back in the next tournament, played in 1945, and reeled off four straight Roca Cups (1945, 1957, 1960, 1963). The two teams shared the title in 1971 before Brazil won the last tournament in 1976, finishing with eight Cups to Argentina's four.