Complex sentence: Prior to Sykes's departure to meet Russian Foreign Minister Sergey Sazonov in Petrograd on 27 February 1916, Sykes was approached with a plan by Samuell in the form of a memorandum which Sykes thought prudent to commit to memory and then destroy.
Split into simpler sentences: Prior to Sykes's departure to meet Russian Foreign Minister Sergey Sazonov in Petrograd on 27 February 1916, Sykes was approached with a plan by Samuel. The plan Samuel put forward was in the form of a memorandum which Sykes thought prudent to commit to memory and then destroy.