Final answer:
The sentences about the history of professional baseball and its segregation era are completed with: Not segregated, Informal agreement, and Played in Negro leagues. Jackie Robinson's 1947 debut was crucial in breaking the color barrier in major league baseball.
Step-by-step explanation:
The best words or phrases to complete each sentence about the history of professional baseball with respect to racial segregation are:
- During its earliest years, professional baseball was Not segregated.
- The segregation of professional baseball was enforced by Informal agreement.
- After the segregation of the major leagues, African American players Played in Negro leagues.
Sadly, the segregation era in baseball history showcases a time when African Americans were excluded from participating alongside white players in the major leagues but managed to carve out their own space and showcase exceptional talent through the Negro leagues, which became popular in their own right with teams like the Kansas City Monarchs and players like Josh Gibson and Satchel Paige. A significant milestone in breaking this segregation was Jackie Robinson's major league debut with the Brooklyn Dodgers in 1947.