Complex sentence: He is suspected of being a member of the "Paxton Boys"--a group of Scots-Irish militants who massacred the Conestoga Indians in 1763--and was a prominent commander on the Yankee (i.e. Connecticut) side in the Pennamite Wars.
Split into simpler sentences: He is suspected of being a member of the "Paxton Boys". A group of Scotts Irish militants who massacred the Conestoga Indians in 1763, and was a prominent military commander--on the Yankee (i.e. Connecticut) side--in the Pennamite Wars.