1927-1950 the globetrotters did play real competitive basketball and were one of the best teams in the world. However, they were based out of Chicago and only used the Harlem moniker to reference their connection to black culture. At first, they would play regional as well as one national tournament called the world professional basketball tournament. Which they won in 1940 and several future NBA teams would go on to win in later years such as the Lakers. Then the NBA became dominate in competitive basketball through the mid 40’s and beyond but did not largely integrate until the late 50’s leaving talented players like Goose Tatum (arguably the best player in the world during his period with the globetrotters) out of the league and barnstorming with the Globetrotters. As the globetrotters lost their true stars to the NBA they shifted to less competitive basketball and more of a show as the WWE would be to Olympic wrestling. They formed the Washington Generals in 1952 to play their opponents on tour and have largely stayed out of real basketball since, outside of a few exhibition games.

In summary, the globetrotters originally were a basketball club on par with the New York Knicks or Minnesota Lakers, however because of their predominance of black players they were not asked to join the NBA or the early leagues that would go on to form it. They created a separate concept from the NBA originally as a showcase of African American Basketball talent through the mid 40’s-50’s, and then as the NBA began integration African American players they transitioned to the fun basketball themed showcase we have today.

Sources Unfortunately, I could not find a serious academic source for the globetrotters and hope a popular history book and website will suffice.

Spinning the globe the rise fall and return to greatness of the Harlem Globetrotters, Ben Green Amistad Jan 3, 2006, reprint

Here’s a nice PBS article, http://www.pbs.org/pov/hardwood/background/

Would like to see a social historian follow up with more about the slow integration of the NBA as compared to baseball.

You honestly might have better luck asking in /r/NBA