Complex sentence: The majority of successful engagements were enabled by the skills of the Selous Scouts (many of which were former enemy); they had the capacity to insert observation posts into the bush without being noticed by the inhabitants.
Split into simpler sentences: The majority of successful engagements were enabled by the skills of the Selous Scouts (many of which were former enemy). They had the capacity to insert observation posts into the bush without being noticed by the inhabitants.