Complex sentence: This was one reason behind Curiel's trip to Paris in March 1937, where the foreign offices of the party was located, and where he formed contacts withn Emilio Sereni, Ambrogio Donini and Ruggiero Grieco.
Split into simpler sentences: This was one reason behind Curiel's trip to Paris, the site of the party's foreign offices, in March, 1937. He formed contacts with Emilio Sereni, Ambrogio Donini and Ruggiero Grieco.