1936-38 was the rise of the Mystery Men, and by 1940 they had started to get organized. The Society of Science and the Sentinels of Magic came together in New York as the Crime Crusaders Club was forming in Cleveland and a rather diverse group the papers named the Squadron of Justice started meeting in Washington DC.
The Sentinels of Magic took on a group of Axis Occultists attempting to raise a being known as Koth as a preemptive strike to keep America out of the War, they were defeated and had to call on other groups to end the threat. Once the Stalker and his Disciples were finished the assembled heroes agreed that banding together for mutual support and the ability to assemble specific individuals against specific threats was a good idea and the Justice Society was born. While still apart of their individual teams the heroes agreed to be called on when their individual skills and specialties were required. Hawkman was the first chairman of the Justice Society of America and they operated out of the Civic City brownstone originally used by the Society of Science.
After America entered the Second World War the Justice Society joined with the Seven Soldiers of Victory, The Freedom Fighters, and a great many independent heroes to form the All-Star Squadron. The younger heroes and sidekicks were part of the Young All-Stars a group originally formed by Iron Munro, Columbia, and Genius Jones.