Baseball was becoming a truly national game
in about 1856,when the expression is first recorded, but at that time the
term really meant baseball as played by a new code of rules introduced by
the New York Knickerbocker Ball Club in 1845. As the game grew even more
popular, people assumed that the national game meant baseball was the
nation's favorite sport and it was used this way; the variation the
national pastime was introduced in the 1920s.