How it unfolded
After the American League declared major status in 1901 and raided National League talent, the leagues fought for two years; a 1903 peace deal set the champions against each other in a best-of-nine. Pittsburgh led 3–1. Deacon Phillippe started five games for the Pirates and won three, but Boston's Cy Young and Bill Dinneen held on and won four straight. In Game 8 Dinneen threw a shutout, allowed four hits and struck out Honus Wagner for the final out. Boston scored three on timely hits in the first and fourth — enough. Attendance was only 7,455, but the "Royal Rooters" singing "Tessie" on the road to rattle Pittsburgh became Series lore. Young went 2–1; Dinneen won three.
Why it still matters
The Series proved the institution could work. The Giants refused a Series in 1904, but from 1905 annual play was locked in and has run every year except the 1994 strike. It was the moment the American League was accepted as an equal major league — and the skeleton of today's thirty-club structure.