Complex sentence: Between 1919 and 1920, the Packers competed against other semi-pro clubs from around Wisconsin and the Midwest, before joining the American Professional Football Association (APFA), the forerunner of today's NFL, in 1921.
Split into simpler sentences: Between 1919 and 1920, the Packers competed against other semi-pro clubs from around Wisconsin and the Midwest. They joined the American Professional Football Association (APFA), the forerunner of today's NFL, in 1921.