Complex sentence: Hull City Association Football Club was founded in June 1904; Previous attempts to found a football club had proved difficult because of the dominance in the city of rugby league teams such as Hull F.C. and Hull Kingston Rovers.
Split into simpler sentences: Hull City Association Football Club was founded in June 1904. For some years previously, attempts had been made to found a football club, but this proved difficult because the city was then dominated by rugby league teams such as Hull and Hull Kingston Rovers.