Wikipedia
"U-S-A!" is a chant of the United States of America's initials popular in expressing American pride and supporting American national sports teams. The chant was documented at the 1972 Summer Olympics in Munich, Germany, during the basketball tournament final between the United States and the Soviet Union, but was popularized after being yelled by fans during the 1980 Winter Olympics, held in Lake Placid, New York, where the U.S. men's ice hockey team defeated the Soviet Union in what became known as the " Miracle on Ice", later moving on to beat Finland for the gold medal. The origin of the cheer is unknown, although there are reports which pre-date the 1972 Olympics. It was chanted during the finals of the 1500 meter event and the long jump at the 1936 Summer Olympics and can be heard in Leni Riefenstahl's Festival of Nations documentary.
The chant can be used in other community events, such as at political rallies.