Complex sentence: He was recruited initially as an informant because he was casting horoscopes for people of interest to MI5, and subsequently because it was believed that Hitler was influenced strongly by astrology, and hence might be likely to choose'lucky' astrological dates for major ventures.
Split into simpler sentences: He was recruited initially as an informant because he was casting horoscopes for people of interest to MI5. However, it was believed that Hitler was influenced strongly by astrology, and hence might be likely to choose'lucky' astrological dates for major ventures.