Complex sentence: However he rejected the participation of the MSI in elections, preferring to take the battle to the streets, and so he left the party in 1960 to form the National Vanguard ("Avanguardia Nazionale") as a street-fighting group.
Split into simpler sentences: However he rejected the participation of the MSI in elections, preferring to take the battle to the streets. He then formed, in 1960, the National Vanguard ("Avanguardia Nazionale") as a street-fighting group.