Complex sentence: Parsons was also responsible for arranging for a census of American chemists, and (when the United States entered the war in April 1917) the selection of chemists who were released from military service to carry out research on projects considered important to the war effort.
Split into simpler sentences: Parsons was also responsible for arranging for a census of American chemists. When the United States entered the war in April 1917, selected chemists were released from military service to carry out research on projects considered important to the war effort.