Complex sentence: Shartouni decided that it was the proper time to carry out the operation, then on the night of September 13,1982, he sneaked onto the second floor of the building housing the Kataeb Party Office in Ashrafieh.
Split into simpler sentences: Shartouni decided that it was the proper time to carry out the operation. On the night of September 13,1982, he sneaked onto the second floor of the building housing the Kataeb Party Office in Ashrafieh.