In the 1920s, professional American Soccer League clubs were allowed to play in the tournament, but following the folding of the original ASL, the Open Cup (then called the Challenge Cup) was only open to amateur clubs.
The United are also one of only two American soccer clubs in history to win a continental title, the other being Los Angeles Galaxy.
The SuperLiga, similar to UEFA's Europa League is the second-highest international soccer competition in North America.
In 2008, the tournament was for the best four overall clubs in MLS, which is how D.C. United qualified, by being the Supporters' Shield winners, and having a 1st place overall record.
It's now for the best MLS and Primera División de México clubs that do not qualify for the CONCACAF Champions League.