Complex sentence: Leaving a sizable force at Fort William Henry to distract Montcalm, he began organizing for the expedition to Quebec, only to be ordered by William Pitt, the Secretary of State responsible for the colonies, to attack Louisbourg first.
Split into simpler sentences: Leaving a sizable force at Fort William Henry to distract Montcalm, he began organizing for the expedition to Quebec. He was then ordered by William Pitt, the Secretary of State responsible for the colonies, to attack Louisbourg first.