Complex sentence: Thomas helped suppress Shays' Rebellion in 1786, which led to him achieving the rank of major, but in the years following, he witnessed and was offended by the continuing persecution of Loyalists.
Split into simpler sentences: Thomas helped suppress Shays' Rebellion in 1786, which led to him achieving the rank of major. In the years following, however, he witnessed and was offended by the continuing persecution of Loyalists.