You're likely referring to Alex Morgan's famous goal against Brazil in the 2011 FIFA Women's World Cup. This goal, which has become known as the "Alex Morgan breakaway", took place on July 10, 2011.
This match was a quarterfinal game between the United States and Brazil. The US won 2-1, advancing to the semi-finals.
While other memorable moments in Alex Morgan's career involve breakaways, this is the one that is most commonly referred to.