Complex sentence: After the British capture of Beausejour, the plan to capture Louisbourg included cutting trade to the Fortress in order to weaken the Fortress and, in turn, weaken the French ability to supply the Mi'kmaq in their warfare against the British.
Split into simpler sentences: After the British capture of Beausejour, the plan to capture Louisbourg included cutting trade to the Fortress. This initiative would both weaken the Fortress and, in turn, weaken the French ability to supply the Mi'kmaq in their warfare against the British.