Robertson had led the National League in home runs in 1916 and 1917, in the peak of the deadball era, with 12 round-trippers in each season. He missed the 1918 season due to the war, and when he returned, the Giants shipped him to the Cubs in mid-1919. He wound up being the Cubs' best hitter that season -- a 132 OPS+ -- and on May 11, 1921 in Philadelphia, he had the game of his life against a woeful Phillies team. Irving Sanborn of the Tribune has the scoop, first noting a familiar name back as manager:
Johnny Evers' Cubs went on a swatting spree today and accomplished a victory that will long be remembered by the miserable Phillies. The Chicagoans tore off an assortment of twenty-one wallops for twenty-nine bases and won by 19 to 8. For a comical exhibition the game could not have been improved on. The affair was a farce right from the start. The Cubs broke forth in the first inning by assaulting Jim Ring for five runs. In the sixth Lefty Baumgartner was requested to come out and quell the uprising, and he did, but not until after the round had netted seven tallies, six hits and two bases on balls. The eighth was another of those crazy rounds, five more Cubs nicking the pan. The ninth netted only one tally.
"Swatting spree." "Comical exhibition." "Quell the uprising." "Nicking the pan." Sportswriting just ain't what it used to be. Robertson went 3-for-6 with three runs scored and six RBI; every player on the team got a hit except leadoff man Max Flack (who scored three runs); everyone scored except pitcher Hippo Vaughn, who went 1-for-5 with one RBI.
The Cubs have scored that many or more runs just 22 other times in the 91 seasons since then. The win brought them to an 11-9 record, but that was about the high point of the season; they lost 11 of their next 15 games and finished far behind the league leaders at 64-89.