Complex sentence: The club spent their early years in the Southern Football League and were crowned champions of the Second Division in their first ever season, before joining the Football League in 1920, becoming the first champions of the newly formed Third Division in 1920-21.
Split into simpler sentences: The club spent their early years in the Southern Football League and were crowned champions of the Second Division in their first ever season. Palace joined the Football League in 1920, becoming the first champions of the newly formed Third Division in 1920-21.