Complex sentence: In January 1917, Lishansky left to Egypt to focus on his activities in Nili, which was not known by the Magen members, and caused ill will towards him, and the decline of HaMagen.
Split into simpler sentences: In January 1917, Lishansky left to Egypt to focus on his activities in Nili, which was not known by the HaMagen members. This caused ill will towards him, and the subsequent decline of HaMagen.