Complex sentence: During the onset of the American Revolution, McDonald was an ardent Whig, but refused an appointment to serve in the Continental Army because he would have been second in command to a colonel lacking prior military experience.
Split into simpler sentences: During the onset of the American Revolution, McDonald was an ardent Whig. Despite his loyalty to the American Revolutionary cause, McDonald refused the appointment because he would have been second in command to a colonel lacking prior military experience.