Complex sentence: Cornwallis negotiated the ill-fated Treaty of 1752 with one band of Mi'kmaq, but was unsuccessful in defeating the Mi'kmaq, whom he thought of as British subjects and, therefore, considered them to be rebels.
Split into simpler sentences: Cornwallis negotiated the ill-fated Treaty of 1752 with one band of Mi'kmaq, but was unsuccessful in defeating the Mi'kmaq. Cornwallis thought of as British subjects and, therefore, considered them to be rebels.