Complex sentence: However, it was first necessary for the French to subdue the Mohawks, protecting themselves from a multi-faceted front against both the English and Mohawks; they also wanted to ensure that the two would not ally against the French.
Split into simpler sentences: However, it was first necessary for the French to subdue the Mohawks, protecting themselves from multiple fronts against both the English and Mohawks. In addition, they wanted to ensure that the two would not ally themselves against the French.